Minimum steps to convert all paths in matrix from top left to bottom right as palindromic paths

Given a matrix mat[][] with N rows and M columns. The task is to find the minimum number of changes required in the matrix such that every path from top left to bottom right is a palindromic path. In a path only right and bottom movements are allowed from one cell to another cell.

Examples:

Input: mat[][] = {{1, 2}, {3, 1}}
Output: 0
Explanation:
Every path in the matrix from top left to bottom right is palindromic.
Paths => {1, 2, 1}, {1, 3, 1}

Input: mat[][] = {{1, 2}, {3, 5}}
Output: 1
Explanation:
Only one change is required for the every path to be palindromic.
That is => mat[1][1] = 1
Paths => {1, 2, 1}, {1, 3, 1}

Approach: The key observation in the problem is that element at same distance from the front end or rear end are equal. Therefore, find all the elements at equal distance from (0, 0) and (N-1, M-1) and then make all of them equal in minimum number of changes. Maintain a count variable to get the total number of changes. Below is the illustration of the approach:



  • Distance possible from the top left and bottom right is 0 to N + M – 2.
  • Maintain two-pointers one at top left that is distance at 0 and another at N + M – 2.
  • Iterate over the matrix and for all distance maintain a hash-map of the elements of the matrix at the current distance.
  • Update the matrix elements with minimum number of changes required.
  • Finally, increment the left distance by 1 and decrement the right distance by 1.

Below is the implementation of the above approach:

C++

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// C++ implementation to find the 
// minimum number of changes required
// such that every path from top left 
// to the bottom right 
// are palindromic paths
  
#include <bits/stdc++.h>
using namespace std;
#define M 3
#define N 3
  
// Function to find the minimum number 
// of the changes required for the 
// every path to be palindromic
int minchanges(int mat[N][M])
{
    // count variable for 
    // maintaining total changes.
    int count = 0;
  
    // left and right variables for 
    // keeping distance values
    // from cell(0, 0) and 
    // (N-1, M-1) respectively.
    int left = 0, right = N + M - 2;
  
    while (left < right) {
  
        unordered_map<int, int> mp;
        int totalsize = 0;
  
        // Iterating over the matrix
        for (int i = 0; i < N; i++) {
            for (int j = 0; j < M; j++) {
                if (i + j == left) {
                    mp[mat[i][j]]++;
                    totalsize++;
                }
                else if (i + j == right) {
                    mp[mat[i][j]]++;
                    totalsize++;
                }
            }
        }
  
        // Finding minimum number
        // of changes required.
        unordered_map<int
          int>::iterator itr = mp.begin();
        int changes = 0;
        for (; itr != mp.end(); itr++)
            changes = max(changes, itr->second);
  
        // Minimum no. of changes will 
        // be the the minimum no.
        // of different values and 
        // we will assume to
        // make them equals to value 
        // with maximum frequency element
        count += totalsize - changes;
          
        // Moving ahead with
        // greater distance
        left++;
        right--;
    }
    return count;
}
  
// Driven Code
int main()
{
    int mat[][M] = {
        { 1, 4, 1 },
        { 2, 5, 3 },
        { 1, 3, 1 }
    };
  
    cout << minchanges(mat);
    return 0;
}

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Output:

2

Performance Analysis:

  • Time Complexity: O(N3)
  • Auxiliary Space: O(N)

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