Java Program to Maximize count of corresponding same elements in given permutations using cyclic rotations

Last Updated : 27 Jan, 2022

Given two permutations P1 and P2 of numbers from 1 to N, the task is to find the maximum count of corresponding same elements in the given permutations by performing a cyclic left or right shift on P1.
Examples:

Input: P1 = [5 4 3 2 1], P2 = [1 2 3 4 5]
Output: 1
Explanation:
We have a matching pair at index 2 for element 3.
Input: P1 = [1 3 5 2 4 6], P2 = [1 5 2 4 3 6]
Output: 3
Explanation:
Cyclic shift of second permutation towards right would give 6 1 5 2 4 3, and we get a match of 5, 2, 4. Hence, the answer is 3 matching pairs.

Naive Approach: The naive approach is to check for every possible shift in both the left and right direction count the number of matching pairs by looping through all the permutations formed.
Time Complexity: O(N²)
Auxiliary Space: O(1)
Efficient Approach: The above naive approach can be optimized. The idea is for every element to store the smaller distance between positions of this element from the left and right sides in separate arrays. Hence, the solution to the problem will be calculated as the maximum frequency of an element from the two separated arrays. Below are the steps:

Store the position of all the elements of the permutation P2 in an array(say store[]).
For each element in the permutation P1, do the following:
- Find the difference(say diff) between the position of the current element in P2 with the position in P1.
- If diff is less than 0 then update diff to (N – diff).
- Store the frequency of current difference diff in a map.
After the above steps, the maximum frequency stored in the map is the maximum number of equal elements after rotation on P1.

Below is the implementation of the above approach:

Java

// Java program for  
// the above approach 
import java.util.*; 
class GFG{ 
  
// Function to maximize the matching 
// pairs between two permutation 
// using left and right rotation 
static int maximumMatchingPairs(int perm1[], 
                                int perm2[], 
                                int n) 
{ 
  // Left array store distance of element 
  // from left side and right array store 
  // distance of element from right side 
  int []left = new int[n]; 
  int []right = new int[n]; 
  
  // Map to store index of elements 
  HashMap<Integer, 
          Integer> mp1 =  new HashMap<>(); 
  HashMap<Integer, 
          Integer> mp2 =  new HashMap<>(); 
    
  for (int i = 0; i < n; i++)  
  { 
    mp1.put(perm1[i], i); 
  } 
  for (int j = 0; j < n; j++)  
  { 
    mp2.put(perm2[j], j); 
  } 
  
  for (int i = 0; i < n; i++)  
  { 
    // idx1 is index of element 
    // in first permutation 
    // idx2 is index of element 
    // in second permutation 
    int idx2 = mp2.get(perm1[i]); 
    int idx1 = i; 
  
    if (idx1 == idx2)  
    { 
      // If element if present on same 
      // index on both permutations then 
      // distance is zero 
      left[i] = 0; 
      right[i] = 0; 
    } 
    else if (idx1 < idx2)  
    { 
      // Calculate distance from left 
      // and right side 
      left[i] = (n - (idx2 - idx1)); 
      right[i] = (idx2 - idx1); 
    } 
    else 
    { 
      // Calculate distance from left 
      // and right side 
      left[i] = (idx1 - idx2); 
      right[i] = (n - (idx1 - idx2)); 
    } 
  } 
  
  // Maps to store frequencies of elements 
  // present in left and right arrays 
  HashMap<Integer, 
          Integer> freq1 = new HashMap<>(); 
  HashMap<Integer, 
          Integer> freq2 = new HashMap<>(); 
    
  for (int i = 0; i < n; i++)  
  { 
    if(freq1.containsKey(left[i])) 
      freq1.put(left[i],  
      freq1.get(left[i]) + 1); 
    else
      freq1.put(left[i], 1); 
    if(freq2.containsKey(right[i])) 
      freq2.put(right[i],  
      freq2.get(right[i]) + 1); 
    else
      freq2.put(right[i], 1); 
  } 
  
  int ans = 0; 
  
  for (int i = 0; i < n; i++)  
  { 
    // Find maximum frequency 
    ans = Math.max(ans,  
          Math.max(freq1.get(left[i]), 
                   freq2.get(right[i]))); 
  } 
  
  // Return the result 
  return ans; 
} 
  
// Driver Code 
public static void main(String[] args) 
{ 
  // Given permutations P1 and P2 
  int P1[] = {5, 4, 3, 2, 1}; 
  int P2[] = {1, 2, 3, 4, 5}; 
  int n = P1.length; 
  
  // Function Call 
  System.out.print(maximumMatchingPairs(P1, P2, n)); 
} 
} 
  
// This code is contributed by gauravrajput1