Count pairs in a binary tree whose sum is equal to a given value x

Last Updated : 12 Apr, 2023

Given a binary tree containing n distinct numbers and a value x. The problem is to count pairs in the given binary tree whose sum is equal to the given value x.

Examples:

Input : 
        5      
       / \      
      3   7      
     / \ / \  
    2  4 6  8   

        x = 10

Output : 3
The pairs are (3, 7), (2, 8) and (4, 6).

1) Naive Approach:

One by one get each node of the binary tree through any of the tree traversals methods. Pass the node say temp, the root of the tree and value x to another function say findPair(). In the function with the help of the root pointer traverse the tree again. One by one sum up these nodes with temp and check whether sum == x. If so, increment count. Calculate count = count / 2 as a single pair has been counted twice by the aforementioned method.

Implementation:

C++

// C++ implementation to count pairs in a binary tree
// whose sum is equal to given value x
#include <bits/stdc++.h>
using namespace std;
 
// structure of a node of a binary tree
struct Node {
    int data;
    Node *left, *right;
};
 
// function to create and return a node
// of a binary tree
Node* getNode(int data)
{
    // allocate space for the node
    Node* new_node = (Node*)malloc(sizeof(Node));
 
    // put in the data
    new_node->data = data;
    new_node->left = new_node->right = NULL;
      return new_node;
}
 
// returns true if a pair exists with given sum 'x'
bool findPair(Node* root, Node* temp, int x)
{
    // base case
    if (!root)
        return false;
 
    // pair exists
    if (root != temp && ((root->data + temp->data) == x))
        return true;
 
    // find pair in left and right subtrees
    if (findPair(root->left, temp, x) || findPair(root->right, temp, x))
        return true;
 
    // pair does not exists with given sum 'x'
    return false;
}
 
// function to count pairs in a binary tree
// whose sum is equal to given value x
void countPairs(Node* root, Node* curr, int x, int& count)
{
    // if tree is empty
    if (!curr)
        return;
 
    // check whether pair exists for current node 'curr'
    // in the binary tree that sum up to 'x'
    if (findPair(root, curr, x))
        count++;
 
    // recursively count pairs in left subtree
    countPairs(root, curr->left, x, count);
 
    // recursively count pairs in right subtree
    countPairs(root, curr->right, x, count);
}
 
// Driver program to test above
int main()
{
    // formation of binary tree
    Node* root = getNode(5);        /*      5         */
    root->left = getNode(3);        /*     / \        */
    root->right = getNode(7);       /*    3   7       */
    root->left->left = getNode(2);  /*   / \ / \      */
    root->left->right = getNode(4); /*   2 4 6 8      */
    root->right->left = getNode(6);
    root->right->right = getNode(8);
 
    int x = 10;
    int count = 0;
 
    countPairs(root, root, x, count);
    count = count / 2;
 
    cout << "Count = " << count;
 
    return 0;
}
// This code is contributed by yash agarwal(yashagarwal2852002)

Java

// Java implementation to count pairs in a binary tree
// whose sum is equal to given value x
import java.util.*;
 
class GFG
{
 
// structure of a node of a binary tree
static class Node 
{
    int data;
    Node left, right;
};
static int count;
 
// function to create and return a node
// of a binary tree
static Node getNode(int data)
{
    // allocate space for the node
    Node new_node = new Node();
 
    // put in the data
    new_node.data = data;
    new_node.left = new_node.right = null;
    return new_node;
}
 
// returns true if a pair exists with given sum 'x'
static boolean findPair(Node root, Node temp, int x)
{
    // base case
    if (root==null)
        return false;
 
    // pair exists
    if (root != temp && ((root.data + temp.data) == x))
        return true;
 
    // find pair in left and right subtrees
    if (findPair(root.left, temp, x) || 
           findPair(root.right, temp, x))
        return true;
 
    // pair does not exists with given sum 'x'
    return false;
}
 
// function to count pairs in a binary tree
// whose sum is equal to given value x
static void countPairs(Node root, Node curr, int x)
{
    // if tree is empty
    if (curr == null)
        return;
 
    // check whether pair exists for current node 'curr'
    // in the binary tree that sum up to 'x'
    if (findPair(root, curr, x))
        count++;
 
    // recursively count pairs in left subtree
    countPairs(root, curr.left, x);
 
    // recursively count pairs in right subtree
    countPairs(root, curr.right, x);
}
 
// Driver code
public static void main(String[] args)
{
    // formation of binary tree
    Node root = getNode(5); /*     5     */
    root.left = getNode(3); /*     / \     */
    root.right = getNode(7); /* 3 7     */
    root.left.left = getNode(2); /* / \ / \ */
    root.left.right = getNode(4); /* 2 4 6 8 */
    root.right.left = getNode(6);
    root.right.right = getNode(8);
 
    int x = 10;
    count = 0;
 
    countPairs(root, root, x);
    count = count / 2;
 
    System.out.print("Count = " + count);
}
}
 
// This code is contributed by PrinciRaj1992

Python3

# Python3 implementation to count pairs in a binary tree
# whose sum is equal to given value x
 
# structure of a node of a binary tree
class getNode(object):
    def __init__(self, value):
        self.data = value
        self.left = None
        self.right = None
 
# returns True if a pair exists with given sum 'x'
def findPair(root, temp, x):
    # base case
    if root == None:
        return False
         
    # pair exists
    if (root != temp and ((root.data + temp.data) == x)):
        return True
         
    # find pair in left and right subtrees
    if (findPair(root.left, temp, x) or findPair(root.right, temp, x)):
        return True
         
    # pair does not exists with given sum 'x'
    return False
     
# function to count pairs in a binary tree
# whose sum is equal to given value x
def countPairs(root, curr, x):
    global count
     
    # if tree is empty
    if curr == None:
        return
     
    # check whether pair exists for current node 'curr'
    # in the binary tree that sum up to 'x'
    if (findPair(root, curr, x)):
        count += 1
         
    # recursively count pairs in left subtree
    countPairs(root, curr.left, x)
     
    # recursively count pairs in right subtree
    countPairs(root, curr.right, x)
 
# Driver program to test above
# formation of binary tree
root = getNode(5)    
root.left = getNode(3) 
root.right = getNode(7) 
root.left.left = getNode(2) 
root.left.right = getNode(4)
root.right.left = getNode(6)
root.right.right = getNode(8)
x = 10
count = 0
 
countPairs(root, root, x)
count = count // 2
 
print("Count =", count)
 
# This code is contributed by shubhamsingh10

C#

// C# implementation to count pairs in a binary tree
// whose sum is equal to given value x
using System;
 
class GFG
{
 
// structure of a node of a binary tree
class Node 
{
    public int data;
    public Node left, right;
};
static int count;
 
// function to create and return a node
// of a binary tree
static Node getNode(int data)
{
    // allocate space for the node
    Node new_node = new Node();
 
    // put in the data
    new_node.data = data;
    new_node.left = new_node.right = null;
    return new_node;
}
 
// returns true if a pair exists with given sum 'x'
static bool findPair(Node root, Node temp, int x)
{
    // base case
    if (root == null)
        return false;
 
    // pair exists
    if (root != temp && ((root.data + temp.data) == x))
        return true;
 
    // find pair in left and right subtrees
    if (findPair(root.left, temp, x) || 
        findPair(root.right, temp, x))
        return true;
 
    // pair does not exists with given sum 'x'
    return false;
}
 
// function to count pairs in a binary tree
// whose sum is equal to given value x
static void countPairs(Node root, Node curr, int x)
{
    // if tree is empty
    if (curr == null)
        return;
 
    // check whether pair exists for current node 'curr'
    // in the binary tree that sum up to 'x'
    if (findPair(root, curr, x))
        count++;
 
    // recursively count pairs in left subtree
    countPairs(root, curr.left, x);
 
    // recursively count pairs in right subtree
    countPairs(root, curr.right, x);
}
 
// Driver code
public static void Main(String[] args)
{
    // formation of binary tree
    Node root = getNode(5); /*     5     */
    root.left = getNode(3); /*     / \     */
    root.right = getNode(7); /* 3 7     */
    root.left.left = getNode(2); /* / \ / \ */
    root.left.right = getNode(4); /* 2 4 6 8 */
    root.right.left = getNode(6);
    root.right.right = getNode(8);
 
    int x = 10;
    count = 0;
 
    countPairs(root, root, x);
    count = count / 2;
 
    Console.Write("Count = " + count);
}
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
 
// Javascript implementation to count
// pairs in a binary tree whose sum is
// equal to given value x
 
// Structure of a node of a binary tree
class Node 
{
    constructor()
    {
        this.data = 0;
        this.left = null;
        this.right = null;
    }
};
var count = 0;
 
// Function to create and return a node
// of a binary tree
function getNode(data)
{
     
    // Allocate space for the node
    var new_node = new Node();
 
    // Put in the data
    new_node.data = data;
    new_node.left = new_node.right = null;
    return new_node;
}
 
// Returns true if a pair exists
// with given sum 'x'
function findPair(root, temp, x)
{
     
    // Base case
    if (root == null)
        return false;
 
    // Pair exists
    if (root != temp && 
       ((root.data + temp.data) == x))
        return true;
 
    // Find pair in left and right subtrees
    if (findPair(root.left, temp, x) || 
        findPair(root.right, temp, x))
        return true;
 
    // Pair does not exists with given sum 'x'
    return false;
}
 
// Function to count pairs in a binary tree
// whose sum is equal to given value x
function countPairs( root, curr, x)
{
     
    // If tree is empty
    if (curr == null)
        return;
 
    // Check whether pair exists for current node
    // 'curr' in the binary tree that sum up to 'x'
    if (findPair(root, curr, x))
        count++;
 
    // Recursively count pairs in left subtree
    countPairs(root, curr.left, x);
 
    // Recursively count pairs in right subtree
    countPairs(root, curr.right, x);
}
 
// Driver code
 
// Formation of binary tree
var root = getNode(5); /*     5     */
root.left = getNode(3); /*     / \     */
root.right = getNode(7); /* 3 7     */
root.left.left = getNode(2); /* / \ / \ */
root.left.right = getNode(4); /* 2 4 6 8 */
root.right.left = getNode(6);
root.right.right = getNode(8);
var x = 10;
count = 0;
 
countPairs(root, root, x);
count = parseInt(count / 2);
 
document.write("Count = " + count);
 
// This code is contributed by noob2000
 
</script>

Output

Count = 3

Time Complexity: O(n^2).

Auxiliary Space: O(1)

2) Efficient Approach: Following are the steps:

Convert given binary tree to doubly linked list. Refer this post.
Sort the doubly linked list obtained in Step 1. Refer this post.
Count Pairs in sorted doubly linked with sum equal to ‘x’. Refer this post.
Display the count obtained in Step 4.

Implementation:

C++

// C++ implementation to count pairs in a binary tree
// whose sum is equal to given value x
#include <bits/stdc++.h>
using namespace std;
 
// structure of a node of a binary tree
struct Node {
    int data;
    Node *left, *right;
    Node(int val){
        this->data = val;
        this->left = this->right = NULL;
    }
};
 
// A simple recursive function to convert a given
// Binary tree to Doubly Linked List
// root     --> Root of Binary Tree
// head_ref --> Pointer to head node of created
// doubly linked list
void BToDLL(Node* root, Node** head_ref)
{
    // Base cases
    if (root == NULL)
        return;
 
    // Recursively convert right subtree
    BToDLL(root->right, head_ref);
 
    // insert root into DLL
    root->right = *head_ref;
 
    // Change left pointer of previous head
    if (*head_ref != NULL)
        (*head_ref)->left = root;
 
    // Change head of Doubly linked list
    *head_ref = root;
 
    // Recursively convert left subtree
    BToDLL(root->left, head_ref);
}
 
// Split a doubly linked list (DLL) into 2 DLLs of
// half sizes
Node* split(Node* head)
{
    Node *fast = head, *slow = head;
    while (fast->right && fast->right->right) {
        fast = fast->right->right;
        slow = slow->right;
    }
    Node* temp = slow->right;
    slow->right = NULL;
    return temp;
}
 
// Function to merge two sorted doubly linked lists
Node* merge(Node* first, Node* second)
{
    // If first linked list is empty
    if (!first)
        return second;
 
    // If second linked list is empty
    if (!second)
        return first;
 
    // Pick the smaller value
    if (first->data < second->data) {
        first->right = merge(first->right, second);
        first->right->left = first;
        first->left = NULL;
        return first;
    }
    else {
        second->right = merge(first, second->right);
        second->right->left = second;
        second->left = NULL;
        return second;
    }
}
 
// Function to do merge sort
Node* mergeSort(Node* head)
{
    if (!head || !head->right)
        return head;
    Node* second = split(head);
 
    // Recur for left and right halves
    head = mergeSort(head);
    second = mergeSort(second);
 
    // Merge the two sorted halves
    return merge(head, second);
}
 
// Function to count pairs in a sorted doubly linked list
// whose sum equal to given value x
int pairSum(Node* head, int x)
{
    // Set two pointers, first to the beginning of DLL
    // and second to the end of DLL.
    Node* first = head;
    Node* second = head;
    while (second->right != NULL)
        second = second->right;
 
    int count = 0;
 
    // The loop terminates when either of two pointers
    // become NULL, or they cross each other (second->right
    // == first), or they become same (first == second)
    while (first != NULL && second != NULL && first != second && second->right != first) {
        // pair found
        if ((first->data + second->data) == x) {
            count++;
 
            // move first in forward direction
            first = first->right;
 
            // move second in backward direction
            second = second->left;
        }
        else {
            if ((first->data + second->data) < x)
                first = first->right;
            else
                second = second->left;
        }
    }
 
    return count;
}
 
// function to count pairs in a binary tree
// whose sum is equal to given value x
int countPairs(Node* root, int x)
{
    Node* head = NULL;
    int count = 0;
 
    // Convert binary tree to
    // doubly linked list
    BToDLL(root, &head);
 
    // sort DLL
    head = mergeSort(head);
 
    // count pairs
    return pairSum(head, x);
}
 
// Driver program to test above
int main()
{
    // formation of binary tree
    Node* root = new Node(5); /*        5      */
    root->left = new Node(3); /*       / \      */
    root->right = new Node(7); /*    3   7      */
    root->left->left = new Node(2); /*   / \ / \   */
    root->left->right = new Node(4); /*   2 4 6 8   */
    root->right->left = new Node(6);
    root->right->right = new Node(8);
 
    int x = 10;
 
    cout << "Count = "
         << countPairs(root, x);
 
    return 0;
}

Java

// Java implementation to count pairs 
// in a binary tree whose sum is equal to
// given value x
class GFG 
{
 
    // structure of a node of a binary tree
    static class Node 
    {
        int data;
        Node left, right;
    };
 
    static Node head_ref;
 
    // function to create and return a node
    // of a binary tree
    static Node getNode(int data) 
    {
        // allocate space for the node
        Node new_node = new Node();
 
        // put in the data
        new_node.data = data;
        new_node.left = new_node.right = null;
        return new_node;
    }
 
    // A simple recursive function to convert 
    // a given Binary tree to Doubly Linked List
    // root -. Root of Binary Tree
    // head_ref -. Pointer to head node of created
    // doubly linked list
    static void BToDLL(Node root) 
    {
        // Base cases
        if (root == null)
            return;
 
        // Recursively convert right subtree
        BToDLL(root.right);
 
        // insert root into DLL
        root.right = head_ref;
 
        // Change left pointer of previous head
        if (head_ref != null)
            head_ref.left = root;
 
        // Change head of Doubly linked list
        head_ref = root;
 
        // Recursively convert left subtree
        BToDLL(root.left);
    }
 
    // Split a doubly linked list (DLL) 
    // into 2 DLLs of half sizes
    static Node split(Node head)
    {
        Node fast = head, slow = head;
        while (fast.right != null && 
               fast.right.right != null) 
        {
            fast = fast.right.right;
            slow = slow.right;
        }
        Node temp = slow.right;
        slow.right = null;
        return temp;
    }
 
    // Function to merge two sorted 
    // doubly linked lists
    static Node merge(Node first, Node second)
    {
        // If first linked list is empty
        if (first == null)
            return second;
 
        // If second linked list is empty
        if (second == null)
            return first;
 
        // Pick the smaller value
        if (first.data < second.data) 
        {
            first.right = merge(first.right,
                                second);
            first.right.left = first;
            first.left = null;
            return first;
        } 
        else
        {
            second.right = merge(first, 
                                 second.right);
            second.right.left = second;
            second.left = null;
            return second;
        }
    }
 
    // Function to do merge sort
    static Node mergeSort(Node head)
    {
        if (head == null || head.right == null)
            return head;
        Node second = split(head);
 
        // Recur for left and right halves
        head = mergeSort(head);
        second = mergeSort(second);
 
        // Merge the two sorted halves
        return merge(head, second);
    }
 
    // Function to count pairs in a sorted 
    // doubly linked list whose sum equal 
    // to given value x
    static int pairSum(Node head, int x)
    {
        // Set two pointers, first to the beginning 
        // of DLL and second to the end of DLL.
        Node first = head;
        Node second = head;
        while (second.right != null)
            second = second.right;
 
        int count = 0;
 
        // The loop terminates when either of two pointers
        // become null, or they cross each other (second.right
        // == first), or they become same (first == second)
        while (first != null && second != null &&
               first != second && second.right != first)
        {
            // pair found
            if ((first.data + second.data) == x)
            {
                count++;
 
                // move first in forward direction
                first = first.right;
 
                // move second in backward direction
                second = second.left;
            } 
            else
            {
                if ((first.data + second.data) < x)
                    first = first.right;
                else
                    second = second.left;
            }
        }
        return count;
    }
 
    // function to count pairs in a binary tree
    // whose sum is equal to given value x
    static int countPairs(Node root, int x) 
    {
        head_ref = null;
 
        // Convert binary tree to
        // doubly linked list
        BToDLL(root);
 
        // sort DLL
        head_ref = mergeSort(head_ref);
 
        // count pairs
        return pairSum(head_ref, x);
    }
 
    // Driver Code
    public static void main(String[] args) 
    {
        // formation of binary tree
        Node root = getNode(5); /* 5 */
        root.left = getNode(3); /* / \ */
        root.right = getNode(7); /* 3 7 */
        root.left.left = getNode(2); /* / \ / \ */
        root.left.right = getNode(4); /* 2 4 6 8 */
        root.right.left = getNode(6);
        root.right.right = getNode(8);
 
        int x = 10;
 
        System.out.print("Count = " + 
                countPairs(root, x));
    }
}
 
// This code is contributed by Rajput-Ji

Python3

# Python3 implementation to count pairs in a binary tree
# whose sum is equal to given value x
  
# structure of a node of a binary tree
class Node:  
    def __init__(self, data):
        self.data = data
        self.left = None
        self.right = None
head_ref = None
  
# function to create and return a node
# of a binary tree
def getNode(data):
 
    # allocate space for the node
    new_node = Node(data)
    return new_node
  
# A simple recursive function to convert a given
# Binary tree to Doubly Linked List
# root     -. Root of Binary Tree
# head_ref -. Pointer to head node of created
# doubly linked list
def BToDLL(root):
    global head_ref
     
    # Base cases
    if (root == None):
        return;
  
    # Recursively convert right subtree
    BToDLL(root.right)
  
    # insert root into DLL
    root.right = head_ref;
  
    # Change left pointer of previous head
    if (head_ref != None):
        (head_ref).left = root;
  
    # Change head of Doubly linked list
    head_ref = root;
  
    # Recursively convert left subtree
    BToDLL(root.left);    
    return head_ref
  
# Split a doubly linked list (DLL) into 2 DLLs of
# half sizes
def split(head):
    fast = head
    slow = head;    
    while (fast.right and fast.right.right):
        fast = fast.right.right;
        slow = slow.right;    
    temp = slow.right;
    slow.right = None;    
    return temp;
  
# Function to merge two sorted doubly linked lists
def merge(first, second):
 
    # If first linked list is empty
    if (not first):
        return second;
  
    # If second linked list is empty
    if (not second):
        return first;
  
    # Pick the smaller value
    if (first.data < second.data):
        first.right = merge(first.right, second);
        first.right.left = first;
        first.left = None;
        return first;
    else:
        second.right = merge(first, second.right);
        second.right.left = second;
        second.left = None;
        return second;
      
# Function to do merge sort
def mergeSort(head):
 
    if (head == None or  head.right == None):
        return head;
    second = split(head);
  
    # Recur for left and right halves
    head = mergeSort(head);
    second = mergeSort(second);
  
    # Merge the two sorted halves
    return merge(head, second);
  
# Function to count pairs in a sorted doubly linked list
# whose sum equal to given value x
def pairSum(head, x):
 
    # Set two pointers, first to the beginning of DLL
    # and second to the end of DLL.
    first = head;
    second = head;    
    while (second.right != None):
        second = second.right;
    count = 0;
  
    # The loop terminates when either of two pointers
    # become None, or they cross each other (second.right
    # == first), or they become same (first == second)
    while (first != None and second != None and first != second and second.right != first):
         
        # pair found
        if ((first.data + second.data) == x):
            count += 1
  
            # move first in forward direction
            first = first.right;
  
            # move second in backward direction
            second = second.left;
         
        else:
            if ((first.data + second.data) < x):
                first = first.right;
            else:
                second = second.left;
    return count;
  
# function to count pairs in a binary tree
# whose sum is equal to given value x
def countPairs(root, x):
    global head_ref    
    head_ref = None;
  
    # Convert binary tree to
    # doubly linked list
    BToDLL(root);
  
    # sort DLL
    head_ref = mergeSort(head_ref);
  
    # count pairs
    return pairSum(head_ref, x);
  
# Driver code
if __name__=='__main__':
     
    # formation of binary tree
    root = getNode(5); #              5      
    root.left = getNode(3); #        / \      
    root.right = getNode(7); #      3   7      
    root.left.left = getNode(2); #  / \ / \   
    root.left.right = getNode(4);# 2  4 6 8   
    root.right.left = getNode(6);
    root.right.right = getNode(8);
    x = 10;
    print("Count = " + str(countPairs(root, x)))
  
# This code is contributed by rutvik_56

C#

// C# implementation to count pairs 
// in a binary tree whose sum is 
// equal to the given value x
using System;
 
class GFG 
{
 
    // structure of a node of a binary tree
    class Node 
    {
        public int data;
        public Node left, right;
    };
 
    static Node head_ref;
 
    // function to create and return a node
    // of a binary tree
    static Node getNode(int data) 
    {
        // allocate space for the node
        Node new_node = new Node();
 
        // put in the data
        new_node.data = data;
        new_node.left = new_node.right = null;
        return new_node;
    }
 
    // A simple recursive function to convert 
    // a given Binary tree to Doubly Linked List
    // root -. Root of Binary Tree
    // head_ref -. Pointer to head node of 
    // created doubly linked list
    static void BToDLL(Node root) 
    {
        // Base cases
        if (root == null)
            return;
 
        // Recursively convert right subtree
        BToDLL(root.right);
 
        // insert root into DLL
        root.right = head_ref;
 
        // Change left pointer of previous head
        if (head_ref != null)
            head_ref.left = root;
 
        // Change head of Doubly linked list
        head_ref = root;
 
        // Recursively convert left subtree
        BToDLL(root.left);
    }
 
    // Split a doubly linked list (DLL) 
    // into 2 DLLs of half sizes
    static Node split(Node head)
    {
        Node fast = head, slow = head;
        while (fast.right != null && 
               fast.right.right != null) 
        {
            fast = fast.right.right;
            slow = slow.right;
        }
        Node temp = slow.right;
        slow.right = null;
        return temp;
    }
 
    // Function to merge two sorted 
    // doubly linked lists
    static Node merge(Node first,
                      Node second)
    {
        // If first linked list is empty
        if (first == null)
            return second;
 
        // If second linked list is empty
        if (second == null)
            return first;
 
        // Pick the smaller value
        if (first.data < second.data) 
        {
            first.right = merge(first.right,
                                second);
            first.right.left = first;
            first.left = null;
            return first;
        } 
        else
        {
            second.right = merge(first, 
                                 second.right);
            second.right.left = second;
            second.left = null;
            return second;
        }
    }
 
    // Function to do merge sort
    static Node mergeSort(Node head)
    {
        if (head == null || head.right == null)
            return head;
        Node second = split(head);
 
        // Recur for left and right halves
        head = mergeSort(head);
        second = mergeSort(second);
 
        // Merge the two sorted halves
        return merge(head, second);
    }
 
    // Function to count pairs in a sorted 
    // doubly linked list whose sum equal 
    // to given value x
    static int pairSum(Node head, int x)
    {
        // Set two pointers, first to the beginning 
        // of DLL and second to the end of DLL.
        Node first = head;
        Node second = head;
        while (second.right != null)
            second = second.right;
 
        int count = 0;
 
        // The loop terminates when either of 
        // two pointers become null, or they 
        // cross each other (second.right == first),
        // or they become same (first == second)
        while (first != null && second != null &&
               first != second && second.right != first)
        {
            // pair found
            if ((first.data + second.data) == x)
            {
                count++;
 
                // move first in forward direction
                first = first.right;
 
                // move second in backward direction
                second = second.left;
            } 
            else
            {
                if ((first.data + second.data) < x)
                    first = first.right;
                else
                    second = second.left;
            }
        }
        return count;
    }
 
    // function to count pairs in a binary tree
    // whose sum is equal to given value x
    static int countPairs(Node root, int x) 
    {
        head_ref = null;
 
        // Convert binary tree to
        // doubly linked list
        BToDLL(root);
 
        // sort DLL
        head_ref = mergeSort(head_ref);
 
        // count pairs
        return pairSum(head_ref, x);
    }
 
    // Driver Code
    public static void Main(String[] args) 
    {
        // formation of binary tree
        Node root = getNode(5);         /* 5 */
        root.left = getNode(3);         /* / \ */
        root.right = getNode(7);     /* 3 7 */
        root.left.left = getNode(2); /* / \ / \ */
        root.left.right = getNode(4); /* 2 4 6 8 */
        root.right.left = getNode(6);
        root.right.right = getNode(8);
 
        int x = 10;
 
        Console.Write("Count = " + 
                countPairs(root, x));
    }
}
 
// This code is contributed by Rajput-Ji

Javascript

<script>
 
      // JavaScript implementation to count pairs
      // in a binary tree whose sum is
      // equal to the given value x
      // structure of a node of a binary tree
      class Node {
        constructor() {
          this.data = 0;
          this.left = null;
          this.right = null;
        }
      }
 
      var head_ref = null;
 
      // function to create and return a node
      // of a binary tree
      function getNode(data) {
        // allocate space for the node
        var new_node = new Node();
 
        // put in the data
        new_node.data = data;
        new_node.left = new_node.right = null;
        return new_node;
      }
 
      // A simple recursive function to convert
      // a given Binary tree to Doubly Linked List
      // root -. Root of Binary Tree
      // head_ref -. Pointer to head node of
      // created doubly linked list
      function BToDLL(root) {
        // Base cases
        if (root == null) return;
 
        // Recursively convert right subtree
        BToDLL(root.right);
 
        // insert root into DLL
        root.right = head_ref;
 
        // Change left pointer of previous head
        if (head_ref != null) head_ref.left = root;
 
        // Change head of Doubly linked list
        head_ref = root;
 
        // Recursively convert left subtree
        BToDLL(root.left);
      }
 
      // Split a doubly linked list (DLL)
      // into 2 DLLs of half sizes
      function split(head) {
        var fast = head,
          slow = head;
        while (fast.right != null && fast.right.right != null) {
          fast = fast.right.right;
          slow = slow.right;
        }
        var temp = slow.right;
        slow.right = null;
        return temp;
      }
 
      // Function to merge two sorted
      // doubly linked lists
      function merge(first, second) {
        // If first linked list is empty
        if (first == null) return second;
 
        // If second linked list is empty
        if (second == null) return first;
 
        // Pick the smaller value
        if (first.data < second.data) {
          first.right = merge(first.right, second);
          first.right.left = first;
          first.left = null;
          return first;
        } else {
          second.right = merge(first, second.right);
          second.right.left = second;
          second.left = null;
          return second;
        }
      }
 
      // Function to do merge sort
      function mergeSort(head) {
        if (head == null || head.right == null) return head;
        var second = split(head);
 
        // Recur for left and right halves
        head = mergeSort(head);
        second = mergeSort(second);
 
        // Merge the two sorted halves
        return merge(head, second);
      }
 
      // Function to count pairs in a sorted
      // doubly linked list whose sum equal
      // to given value x
      function pairSum(head, x) {
        // Set two pointers, first to the beginning
        // of DLL and second to the end of DLL.
        var first = head;
        var second = head;
        while (second.right != null) second = second.right;
 
        var count = 0;
 
        // The loop terminates when either of
        // two pointers become null, or they
        // cross each other (second.right == first),
        // or they become same (first == second)
        while (
          first != null &&
          second != null &&
          first != second &&
          second.right != first
        ) {
          // pair found
          if (first.data + second.data == x) {
            count++;
 
            // move first in forward direction
            first = first.right;
 
            // move second in backward direction
            second = second.left;
          } else {
            if (first.data + second.data < x) first = first.right;
            else second = second.left;
          }
        }
        return count;
      }
 
      // function to count pairs in a binary tree
      // whose sum is equal to given value x
      function countPairs(root, x) {
        head_ref = null;
 
        // Convert binary tree to
        // doubly linked list
        BToDLL(root);
 
        // sort DLL
        head_ref = mergeSort(head_ref);
 
        // count pairs
        return pairSum(head_ref, x);
      }
 
      // Driver Code
      // formation of binary tree
      var root = getNode(5); /* 5 */
      root.left = getNode(3); /* / \ */
      root.right = getNode(7); /* 3 7 */
      root.left.left = getNode(2); /* / \ / \ */
      root.left.right = getNode(4); /* 2 4 6 8 */
      root.right.left = getNode(6);
      root.right.right = getNode(8);
 
      var x = 10;
 
      document.write("Count = " + countPairs(root, x));
    </script>

Output

Count = 3

Time Complexity: O(nLog n).

Auxiliary Space: O(N)

3)Another Efficient Approach – No need for converting to DLL and sorting: Following are the steps:

Traverse the tree in any order (pre / post / in).
Create an empty hash and keep adding difference between current node’s value and X to it.
At each node, check if it’s value is in the hash, if yes then increment the count by 1 and DO NOT add this node’s value’s difference with X in the hash to avoid duplicate counting for a single pair.

Implementation:

C++

#include <iostream>
#include <unordered_set>
using namespace std;
 
// Node class to represent a
// node in the binary tree
// with value, left and right attributes
class Node {
public:
    int value;
    Node* left;
    Node* right;
 
    Node(int value, Node* left = nullptr, Node* right = nullptr) : value(value), left(left), right(right) {}
};
 
// To store count of pairs
int count = 0;
 
// To store difference between
// current node's value and x,
// acts a lookup for counting pairs
unordered_set<int> hash_t;
 
// The input, we need to count
// pairs whose sum is equal to x
int x = 10;
 
// Function to count number of pairs
// Does a pre-order traversal of the tree
void count_pairs_w_sum(Node* root) {
    if (root != nullptr) {
        if (hash_t.count(root->value)) {
            count++;
        }
        else {
            hash_t.insert(x - root->value);
        }
 
        count_pairs_w_sum(root->left);
        count_pairs_w_sum(root->right);
    }
}
 
// Entry point / Driver - Create a
// binary tree and call the function
// to get the count
int main() {
    Node* root = new Node(5);
 
    root->left = new Node(3);
    root->right = new Node(7);
 
    root->left->left = new Node(2);
    root->left->right = new Node(4);
 
    root->right->left = new Node(6);
    root->right->right = new Node(8);
 
    count_pairs_w_sum(root);
 
    cout << count << endl;
 
    return 0;
}
 
// This code is contributed by lokeshpotta20.

Java

// Java program to Count pairs
// in a binary tree whose sum is
// equal to a given value x
import java.util.HashSet;
 
public class GFG
{
 
  // Node class to represent a
  // node in the binary tree
  // with value, left and right attributes
  static class Node {
    int value;
    Node left, right;
    public Node(int value) {
      this.value = value;
    }
  }    
 
  // To store count of pairs
  static int count = 0;
 
  // To store difference between
  // current node's value and x,
  // acts a lookup for counting pairs
  static HashSet<Integer> hash_t = new HashSet<Integer>();
 
  // The input, we need to count
  // pairs whose sum is equal to x
  static int x = 10;
 
  // Function to count number of pairs
  // Does a pre-order traversal of the tree
  static void count_pairs_w_sum(Node root) {
    if( root != null) {
      if (hash_t.contains(root.value))
        count += 1;
      else
        hash_t.add(x-root.value);
 
      count_pairs_w_sum(root.left);
      count_pairs_w_sum(root.right);
    }
  }
 
 
  //Driver method
  public static void main(String[] args) {
    Node root = new Node(5);
 
    root.left = new Node(3);
    root.right = new Node(7);
 
    root.left.left = new Node(2);
    root.left.right = new Node(4);
 
    root.right.left = new Node(6);
    root.right.right = new Node(8);
 
    count_pairs_w_sum(root);
 
    System.out.println(count);
  }
}
 
// This code is contributed by Lovely Jain

Python

# Python program to Count pairs
# in a binary tree whose sum is
# equal to a given value x
 
# Node class to represent a
# node in the binary tree
# with value, left and right attributes
class Node(object):
    def __init__(self, value, left=None, right=None):
        self.value = value
        self.left = left
        self.right = right
 
 
# To store count of pairs
count = 0
 
# To store difference between
# current node's value and x,
# acts a lookup for counting pairs
hash_t = set()
 
# The input, we need to count
# pairs whose sum is equal to x
x = 10
 
# Function to count number of pairs
# Does a pre-order traversal of the tree
 
 
def count_pairs_w_sum(root):
    global count
    if root:
        if root.value in hash_t:
            count += 1
        else:
            hash_t.add(x-root.value)
 
        count_pairs_w_sum(root.left)
        count_pairs_w_sum(root.right)
 
 
# Entry point / Driver - Create a
# binary tree and call the function
# to get the count
if __name__ == '__main__':
    root = Node(5)
 
    root.left = Node(3)
    root.right = Node(7)
 
    root.left.left = Node(2)
    root.left.right = Node(4)
 
    root.right.left = Node(6)
    root.right.right = Node(8)
 
    count_pairs_w_sum(root)
 
    print count

C#

// C# program to count pairs in binary tree whose sum is
// equal to a given value in x
 
using System;
using System.Collections.Generic;
 
public class GFG {
 
    // Node class to represent a
    // node in the binary tree
    // with value, left and right attributes
    class Node {
        public int value;
        public Node left, right;
        public Node(int value) { this.value = value; }
    }
 
    // To stsore count of pairs
    static int count = 0;
 
    // To store difference between
    // current node's value and x,
    // acts a lookup for counting pairs
    static HashSet<int> hash_t = new HashSet<int>();
 
    // The input, we need to count
    // pairs whose sum is equal to x
    static int x = 10;
 
    // Function to count number of pairs
    // Does a pre-order traversal of the tree
    static void count_pairs_w_sum(Node root)
    {
        if (root != null) {
            if (hash_t.Contains(root.value))
                count += 1;
            else
                hash_t.Add(x - root.value);
 
            count_pairs_w_sum(root.left);
            count_pairs_w_sum(root.right);
        }
    }
 
    static public void Main()
    {
 
        // Code
        Node root = new Node(5);
 
        root.left = new Node(3);
        root.right = new Node(7);
 
        root.left.left = new Node(2);
        root.left.right = new Node(4);
 
        root.right.left = new Node(6);
        root.right.right = new Node(8);
 
        count_pairs_w_sum(root);
 
        Console.WriteLine(count);
    }
}
 
// This code is contributed by lokeshmvs21.

Javascript

// THIS CODE IS CONTRIBUTED BY KIRTI AGARWAL
// JavaScript program to count pairs in binary tree whose sum is
// equal to a given value in x
class Node{
    constructor(value){
        this.value = value;
        this.left = null;
        this.right = null;
    }
}
 
// To store count of pairs
let count = 0;
 
// To store difference between
// current node's value and x,
// acts a lookup for counting pairs
let hash_t = new Set();
 
// The input, we need to count
// pairs whose sum is equal to x
let x = 10;
 
// Function to count number of pairs
// Does a pre-order traversal of the tree
function count_pairs_w_sum(root){
    if(root != null){
        if(hash_t.has(root.value)){
            count++;
        }
        else{
            hash_t.add(x-root.value);
        }
         
        count_pairs_w_sum(root.left);
        count_pairs_w_sum(root.right);
    }
}
 
// Entry point / Driver - Create a
// binary tree and call the function
// to get the count
let root = new Node(5);
root.left = new Node(3);
root.right = new Node(7);
 
root.left.left = new Node(2);
root.left.right = new Node(4);
 
root.right.left = new Node(6);
root.right.right = new Node(8);
 
count_pairs_w_sum(root);
 
console.log(count);

Output

Complexity Analysis:

Time Complexity: O(n)
Space Complexity: O(n)

Suggest improvement

Count pairs from two BSTs whose sum is equal to a given value x

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