Find all unique pairs of maximum and second maximum elements over all sub-arrays in O(NlogN)

Last Updated : 24 Mar, 2023

Let $(a, b)$ represent the ordered pair of the second maximum and the maximum element of an array respectively. We need to find all such unique pairs overall contiguous sub-arrays of a given array.

Examples:

Input: Arr = [ 1, 2, 3, 4, 5 ]
Output: (1, 2) (2, 3) (3, 4) (4, 5)

Input: Arr = [ 1, 1, 2 ]
Output: (1, 1) (1, 2)

Input: Arr = [ 1, 2, 6, 4, 5 ]
Output: (1, 2) (2, 6) (4, 5) (4, 6) (5, 6)

Brute Force Approach:

A simple way to solve this problem would be to scan each sub-array and find the maximum and second maximum element in that sub-array
This can be done in $O(N^2)$ time
Then we can insert each pair in a set to ensure duplicates are removed, and then print them
Each insertion operation costs $O(log(N))$ , pushing the final complexity to $O(N^2log(N))$

C++14

// C++ implementation 
#include <bits/stdc++.h> 
using namespace std; 
  
// Function to return the set of pairs 
set<pair<int, int> > pairs(vector<int>& arr) 
{ 
    set<pair<int, int> > pairs; 
  
    // find all subarrays 
    for (int i = 0; i < arr.size() - 1; ++i) { 
        int maximum = max(arr[i], arr[i + 1]), 
            secondmax = min(arr[i], arr[i + 1]); 
  
        for (int j = i + 1; j < arr.size(); ++j) { 
            // update max and second max 
            if (arr[j] > maximum) { 
                secondmax = maximum; 
                maximum = arr[j]; 
            } 
            if (arr[j] < maximum && arr[j] > secondmax) { 
                secondmax = arr[j]; 
            } 
  
            // insert a pair in set 
            pairs.insert(make_pair(secondmax, maximum)); 
        } 
    } 
    return pairs; 
} 
  
int main() 
{ 
    vector<int> vec = { 1, 2, 6, 4, 5 }; 
  
    set<pair<int, int> > st = pairs(vec); 
    cout << "Total Number of valid pairs is :"
         << (int)st.size() << "\n"; 
    for (auto& x : st) { 
        cout << "(" << x.first << ", " << x.second << ") "; 
    } 
    return 0; 
}

Java

// Java implementation 
import java.util.HashSet; 
import java.util.Set; 
  
class Pair implements Comparable<Pair> { 
    int first, second; 
  
    public Pair(int first, int second) 
    { 
        this.first = first; 
        this.second = second; 
    } 
  
    @Override public int hashCode() 
    { 
        return 31 * first + second; 
    } 
  
    public boolean equals(Object p) 
    { 
        Pair pair = (Pair)p; 
  
        if (this.first != pair.first) 
            return false; 
  
        return this.second == pair.second; 
    } 
  
    @Override public int compareTo(Pair p) 
    { 
        if (this.first == p.first) { 
            return this.second - p.second; 
        } 
        return this.first - p.first; 
    } 
} 
  
public class GFG { 
  
    // Function to return the set of pairs 
    static Set<Pair> pairs(int[] arr) 
    { 
        Set<Pair> pairs = new HashSet<>(); 
  
        // Find all subarrays 
        for (int i = 0; i < arr.length - 1; ++i) { 
            int maximum = Math.max(arr[i], arr[i + 1]), 
                secondmax = Math.min(arr[i], arr[i + 1]); 
  
            for (int j = i + 1; j < arr.length; ++j) { 
  
                // Update max and second max 
                if (arr[j] > maximum) { 
                    secondmax = maximum; 
                    maximum = arr[j]; 
                } 
                if (arr[j] < maximum 
                    && arr[j] > secondmax) { 
                    secondmax = arr[j]; 
                } 
  
                // Insert a pair in set 
                pairs.add(new Pair(secondmax, maximum)); 
            } 
        } 
        return pairs; 
    } 
  
    // Driver Code 
    public static void main(String[] args) 
    { 
        int[] vec = { 1, 2, 6, 4, 5 }; 
  
        Set<Pair> st = pairs(vec); 
        System.out.println("Total Number of "
                           + "valid pairs is :"
                           + st.size()); 
  
        for (Pair x : st) { 
            System.out.printf("(%d, %d)\n", x.first, 
                              x.second); 
        } 
    } 
} 
  
// This code is contributed by sanjeev2552

Python3

# python3 implementation 
  
# Function to return the set of pairs 
def SetofPairs(arr): 
  
    pairs = set() 
    n = len(arr)     # length of array 
      
    # find all subarrays 
    for i in range(n - 1): 
        maximum = max(arr[i], arr[i + 1]) 
        secondmax = min(arr[i], arr[i + 1]) 
        for j in range(i + 1, n): 
  
            # update max and second max 
            if (arr[j] > maximum): 
                secondmax = maximum 
                maximum = arr[j] 
            if (arr[j] < maximum and arr[j] > secondmax): 
                secondmax = arr[j] 
  
            # add a pair in set 
            pairs.add((secondmax, maximum)) 
    return pairs 
  
# Driver code 
if __name__ == "__main__": 
  
    vec = [1, 2, 6, 4, 5] 
    st = SetofPairs(vec) 
    print("Total Number of valid pairs is :", len(st)) 
  
    for x in st: 
        print(x, end = " ") 
  
        # This code is contributed by sunilsoni10220001022000.

Javascript

<script> 
  
// JavaScript implementation 
  
// Function to return the set of pairs 
function pairs(arr) 
{ 
    var pairs = new Set(); 
  
    // find all subarrays 
    for (var i = 0; i < arr.length - 1; ++i) { 
        var maximum = Math.max(arr[i], arr[i + 1]), 
            secondmax = Math.min(arr[i], arr[i + 1]); 
  
        for (var j = i + 1; j < arr.length; ++j) { 
            // update max and second max 
            if (arr[j] > maximum) { 
                secondmax = maximum; 
                maximum = arr[j]; 
            } 
            if (arr[j] < maximum && arr[j] > secondmax) { 
                secondmax = arr[j]; 
            } 
  
            // insert a pair in set 
            pairs.add([secondmax, maximum].toString()); 
        } 
    } 
    return pairs; 
} 
  
var vec = [1, 2, 6, 4, 5 ]; 
var st = pairs(vec); 
document.write( "Total Number of valid pairs is :" + 
st.size + "<br>"); 
[...st].sort().forEach(x => { 
    x = x.split(','); 
    document.write( "(" + x[0] + ", " + x[1] + ") ") 
}) 
  
</script>

C#

// C# implementation 
using System; 
using System.Collections.Generic; 
  
public class Pair : IComparable<Pair> { 
    public int first, second; 
  
    public Pair(int first, int second) 
    { 
        this.first = first; 
        this.second = second; 
    } 
  
    public override int GetHashCode() 
    { 
        return 31 * first + second; 
    } 
  
    public override bool Equals(Object p) 
    { 
        Pair pair = (Pair)p; 
  
        if (this.first != pair.first) 
            return false; 
  
        return this.second == pair.second; 
    } 
  
    public int CompareTo(Pair p) 
    { 
        if (this.first == p.first) { 
            return this.second - p.second; 
        } 
        return this.first - p.first; 
    } 
} 
  
public class GFG { 
  
    // Function to return the set of pairs 
    static HashSet<Pair> Pairs(int[] arr) 
    { 
        HashSet<Pair> pairs = new HashSet<Pair>(); 
  
        // Find all subarrays 
        for (int i = 0; i < arr.Length - 1; ++i) { 
            int maximum = Math.Max(arr[i], arr[i + 1]), 
                secondmax = Math.Min(arr[i], arr[i + 1]); 
  
            for (int j = i + 1; j < arr.Length; ++j) { 
  
                // Update max and second max 
                if (arr[j] > maximum) { 
                    secondmax = maximum; 
                    maximum = arr[j]; 
                } 
                if (arr[j] < maximum 
                    && arr[j] > secondmax) { 
                    secondmax = arr[j]; 
                } 
  
                // Insert a pair in set 
                pairs.Add(new Pair(secondmax, maximum)); 
            } 
        } 
        return pairs; 
    } 
  
    // Driver Code 
    public static void Main(String[] args) 
    { 
        int[] vec = { 1, 2, 6, 4, 5 }; 
  
        HashSet<Pair> st = Pairs(vec); 
        Console.WriteLine("Total Number of " + 
            "valid pairs is :" + st.Count); 
  
        foreach (Pair x in st) { 
            Console.WriteLine("({0}, {1})", 
                x.first, x.second); 
        } 
    } 
}

Output

Total Number of valid pairs is :5
(1, 2) (2, 6) (4, 5) (4, 6) (5, 6)

Complexity Analysis:

Time Complexity: O(N^2 log(N)).
Insertion in set takes log N time. There can be at most N^2 sub-arrays. So the time Complexity is O(N^2 log N).
Auxiliary Space: O(n^2).
As extra space is required to store the elements in a set.

Efficient Approach:

It could bring down the complexity of finding pairs to $O(N)$ by observing that an element $X$ can form pairs with elements only till the closest element to the right which is greater than $X$ .
To see why this holds, consider $X$ = $4$ in the next example.

 Arr = {1, 4, 5, 3, 2, 1}

It could see that $5 > 4$ is the nearest element to the right which is greater than $4$ . $(4, 5)$ forms a pair considering the sub-array $[4, 5]$ .
Other sub-arrays, that start with $4$ must include $5$ . Considering one of them, if another element $Y >=5$ exists in the sub-array, then $(5, Y)$ will be the pair for that sub-array.
Else either $(4, 5)$ will be formed or $(Z, 5)$ will be formed, where $Z$ is the max element to the right of $5$ in the sub-array.
In any cases, $4$ cannot form a pair with any element to the right of $5$ .
Using this observation, we can implement the logic using stack which brings down the pair generation complexity to $O(N)$ .
Each pair can be inserted into a set for eliminating duplicates, giving a final time complexity of $O(Nlog(N))$

Below is the implementation of the above approach:

C++

// C++ implementation of the above approach 
#include <bits/stdc++.h> 
using namespace std; 
  
// Function to return the set of pairs 
set<pair<int, int>>  
     pairs(vector<int>& arr) 
{ 
    stack<int> st; 
    set<pair<int, int>> pairs; 
  
    // Push first element into stack 
    st.push(arr[0]); 
  
    // For each element 'X' in arr, 
    // pop the stack while top Element 
    // is smaller than 'X' and form a pair. 
    // If the stack is not empty after 
    // the previous operation, create 
    // a pair. Push X into the stack. 
  
    for (int i = 1; i < arr.size(); ++i) { 
        while (!st.empty() && 
                arr[i] > st.top()) { 
            pairs.insert(make_pair(st.top(), 
                                    arr[i])); 
            st.pop(); 
        } 
        if (!st.empty()) { 
            pairs.insert(make_pair(min(st.top(), 
                                       arr[i]), 
                                   max(st.top(), 
                                      arr[i]))); 
        } 
        st.push(arr[i]); 
    } 
    return pairs; 
} 
  
int main() 
{ 
    vector<int> vec = { 1, 2, 6, 4, 5 }; 
  
    set<pair<int, int> > st = pairs(vec); 
    cout << "Total Number of valid pairs is :" 
                   << (int)st.size() << "\n"; 
    for (auto& x : st) { 
        cout << "(" << x.first << ", "
                       << x.second << ") "; 
    } 
    return 0; 
} 

Java

// Java implementation of the above approach 
import java.util.*; 
  
public class GFG { 
  
    static class pair { 
        int first, second; 
  
        public pair(int first, int second) 
        { 
            this.first = first; 
            this.second = second; 
        } 
  
        @Override public int hashCode() 
        { 
            final int prime = 31; 
            int result = 1; 
            result = prime * result + first; 
            result = prime * result + second; 
            return result; 
        } 
  
        @Override public boolean equals(Object obj) 
        { 
            if (this == obj) 
                return true; 
            if (obj == null) 
                return false; 
            if (getClass() != obj.getClass()) 
                return false; 
  
            pair other = (pair)obj; 
            if (first != other.first) 
                return false; 
            if (second != other.second) 
                return false; 
            return true; 
        } 
    } 
  
    // Function to return the set of pairs 
    static HashSet<pair> pairs(int[] arr) 
    { 
        Stack<Integer> st = new Stack<Integer>(); 
        HashSet<pair> pairs = new HashSet<pair>(); 
  
        // Push first element into stack 
        st.add(arr[0]); 
  
        // For each element 'X' in arr, 
        // pop the stack while top Element 
        // is smaller than 'X' and form a pair. 
        // If the stack is not empty after 
        // the previous operation, create 
        // a pair. Push X into the stack. 
        for (int i = 1; i < arr.length; ++i) { 
            while (!st.isEmpty() && arr[i] > st.peek()) { 
                pairs.add(new pair(st.peek(), arr[i])); 
                st.pop(); 
            } 
            if (!st.isEmpty()) { 
                pairs.add( 
                    new pair(Math.min(st.peek(), arr[i]), 
                             Math.max(st.peek(), arr[i]))); 
            } 
            st.add(arr[i]); 
        } 
        return pairs; 
    } 
  
    // Driver code 
    public static void main(String[] args) 
    { 
        int[] vec = { 1, 2, 6, 4, 5 }; 
  
        HashSet<pair> st = pairs(vec); 
        System.out.print("Total Number of valid pairs is :"
                         + (int)st.size() + "\n"); 
        for (pair x : st) { 
            System.out.print("(" + x.first + ", " + x.second 
                             + ") "); 
        } 
    } 
} 
  
// This code is contributed by 29AjayKumar

Python3

# Python3 implementation of the above approach 
  
# Function to return the set of pairs  
def pairs(arr) : 
  
    st = [];  
    pairs = [];  
  
    # Push first element into stack  
    st.append(arr[0]);  
  
    # For each element 'X' in arr,  
    # pop the stack while top Element  
    # is smaller than 'X' and form a pair.  
    # If the stack is not empty after  
    # the previous operation, create  
    # a pair. Push X into the stack.  
    for i in range(1, len(arr) ) : 
        while len(st) != 0 and arr[i] > st[-1] : 
            pairs.append((st[-1], arr[i]));  
            st.pop();  
      
        if len(st) != 0 : 
            pairs.append((min(st[-1], arr[i]),  
                        max(st[-1], arr[i])));  
          
        st.append(arr[i]);  
      
    return pairs;  
  
# Driver code 
if __name__ == "__main__" :  
  
    vec = [ 1, 2, 6, 4, 5 ]; 
    st = pairs(vec); 
    print("Total Number of valid pairs is :",len(st));  
      
    for x in st : 
        print("(" ,x[0], ", ",x[1], ")",end=" ");  
  
# This code is contributed by AnkitRai01 

C#

using System; 
using System.Collections.Generic; 
  
class GFG 
{ 
    // Class to store the pair 
    public class pair 
    { 
        public int first, second; 
  
        public pair(int first, int second) 
        { 
            this.first = first; 
            this.second = second; 
        } 
  
        public override int GetHashCode() 
        { 
            int prime = 31; 
            int result = 1; 
            result = prime * result + first; 
            result = prime * result + second; 
            return result; 
        } 
  
        public override bool Equals(object obj) 
        { 
            if (this == obj) 
                return true; 
            if (obj == null) 
                return false; 
            if (GetType() != obj.GetType()) 
                return false; 
  
            pair other = (pair)obj; 
            if (first != other.first) 
                return false; 
            if (second != other.second) 
                return false; 
            return true; 
        } 
    } 
  
    // Function to return the set of pairs 
    static HashSet<pair> pairs(int[] arr) 
    { 
        Stack<int> st = new Stack<int>(); 
        HashSet<pair> pairs = new HashSet<pair>(); 
  
        // Push first element into stack 
        st.Push(arr[0]); 
  
        // For each element 'X' in arr, 
        // pop the stack while top Element 
        // is smaller than 'X' and form a pair. 
        // If the stack is not empty after 
        // the previous operation, create 
        // a pair. Push X into the stack. 
        for (int i = 1; i < arr.Length; ++i) 
        { 
            while (st.Count != 0 && arr[i] > st.Peek()) 
            { 
                pairs.Add(new pair(st.Peek(), 
                                   arr[i])); 
                st.Pop(); 
            } 
            if (st.Count != 0) 
            { 
                pairs.Add(new pair(Math.Min(st.Peek(), 
                                            arr[i]), 
                                   Math.Max(st.Peek(), 
                                            arr[i]))); 
            } 
            st.Push(arr[i]); 
        } 
        return pairs; 
    } 
  
    // Driver code 
    public static void Main(String[] args) 
    { 
        int[] vec = { 1, 2, 6, 4, 5 }; 
  
        HashSet<pair> st = pairs(vec); 
        Console.Write("Total Number of valid pairs is :" + 
                      (int)st.Count + "\n"); 
        foreach (pair x in st) 
        { 
            Console.Write("(" + x.first + ", " + 
                                x.second + ") "); 
        } 
    } 
}

Javascript

<script> 
  
// Javacript implementation of the above approach 
  
function pairs(arr) { 
    let st = []; 
    let pairs = new Set(); 
  
    // Push first element into stack 
    st.push(arr[0]); 
  
    // For each element 'X' in arr, 
    // pop the stack while top Element 
    // is smaller than 'X' and form a pair. 
    // If the stack is not empty after 
    // the previous operation, create 
    // a pair. Push X into the stack. 
  
    for (let i = 1; i < arr.length; ++i) { 
        while (st.length !== 0 && arr[i] > st[st.length - 1]) { 
            pairs.add([st[st.length - 1], arr[i]]); 
            st.pop(); 
        } 
        if (st.length !== 0) { 
            pairs.add([(Math.min(st[st.length - 1], arr[i]), Math.max(st[st.length - 1], arr[i]))]); 
        } 
        st.push(arr[i]); 
    } 
    return pairs; 
} 
  
let vec = [1, 2, 6, 4, 5]; 
let st = pairs(vec); 
console.log(`Total Number of valid pairs is : ${st.size}`); 
for (let x of st) { 
    console.log(`(${x[0]}, ${x[1]})`); 
} 
  
// this code is contributed by bhardwajji 
  
</script>

Output

Total Number of valid pairs is :5
(1, 2) (2, 6) (4, 5) (4, 6) (5, 6)

Complexity Analysis:

Time Complexity: O(N log(N)).
Each pair can be inserted into a set for eliminating duplicates, giving a final time complexity of O(N log N)
Auxiliary Space: O(N).
As extra space is required to store the elements in a set.

Suggest improvement

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Share your thoughts in the comments

Find all unique pairs of maximum and second maximum elements over all sub-arrays in O(NlogN)

C++14

Java

Python3

Javascript

C#

C++

Java

Python3

C#

Javascript

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