Javascript 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:

Javascript

<script> 
  
// Javascript program for the above approach 
  
// Function to maximize the matching 
// pairs between two permutation 
// using left and right rotation 
function maximumMatchingPairs(perm1, perm2, n) 
{ 
    // Left array store distance of element 
    // from left side and right array store 
    // distance of element from right side 
    var left = Array(n); 
    var right = Array(n); 
  
    // Map to store index of elements 
    var mp1 = new Map(), mp2 = new Map(); 
    for (var i = 0; i < n; i++) { 
        mp1.set(perm1[i], i); 
    } 
    for (var j = 0; j < n; j++) { 
        mp2.set(perm2[j], j); 
    } 
  
    for (var i = 0; i < n; i++) { 
  
        // idx1 is index of element 
        // in first permutation 
  
        // idx2 is index of element 
        // in second permutation 
        var idx2 = mp2.get(perm1[i]); 
        var 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 
    var freq1 = new Map(), freq2 = new Map(); 
    for (var i = 0; i < n; i++) { 
        if(freq1.has(left[i])) 
            freq1.set(left[i], freq1.get(left[i])+1) 
        else
            freq1.set(left[i], 1) 
  
        if(freq2.has(right[i])) 
            freq2.set(right[i], freq2.get(right[i])+1) 
        else
            freq2.set(right[i], 1) 
    } 
  
    var ans = 0; 
  
    for (var 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 
// Given permutations P1 and P2 
var P1 = [5, 4, 3, 2, 1]; 
var P2 = [1, 2, 3, 4, 5]; 
var n = P1.length; 
// Function Call 
document.write( maximumMatchingPairs(P1, P2, n)); 
  
</script>