Java Program to Interchange Diagonals of Matrix

Given a square matrix of order n*n, you have to interchange the elements of both diagonals. 
Examples :

Input : matrix[][] = {1, 2, 3,
                      4, 5, 6,
                      7, 8, 9} 
Output : matrix[][] = {3, 2, 1,
                       4, 5, 6,
                       9, 8, 7} 

Input : matrix[][] = {4,  2,  3,  1,
                      5,  7,  6,  8,
                      9, 11, 10, 12,
                     16, 14, 15, 13} 
Output : matrix[][] = {1,  2,  3,  4,
                       5,  6,  7,  8,
                       9, 10, 11, 12,
                      11, 14, 15, 16}

Explanation : Idea behind interchanging diagonals of a square matrix is simple. Iterate from 0 to n-1 and for each iteration you have to swap a[i][i] and a[i][n-i-1]. 

Java

// Java program to interchange
// the diagonals of matrix
import java.io.*;

class GFG
{
    public static int N = 3;

    // Function to interchange diagonals
    static void interchangeDiagonals(int array[][])
    {
        // swap elements of diagonal
        for (int i = 0; i < N; ++i)
            if (i != N / 2)
            {
                int temp = array[i][i];
                array[i][i] = array[i][N - i - 1];
                array[i][N - i - 1] = temp;
            }

        for (int i = 0; i < N; ++i)
        {
            for (int j = 0; j < N; ++j)
                System.out.print(array[i][j]+" ");
            System.out.println();
        }
    }

    // Driver Code
    public static void main (String[] args)
    {
        int array[][] = { {4, 5, 6},
                        {1, 2, 3},
                        {7, 8, 9}
                        };
        interchangeDiagonals(array);
    }
}

// This code is contributed by Pramod Kumar

Output:

 6 5 4
 1 2 3
 9 8 7

Time Complexity: O(N) where N is no of rows or columns; as we are using a single loop for interchanging diagonals of a given matrix.

Auxiliary Space: O(1), as we are not using any extra space.

Please refer complete article on Program to Interchange Diagonals of Matrix for more details!