Python 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]. 

Python3

# Python program to interchange
# the diagonals of matrix
N = 3;

# Function to interchange diagonals
def interchangeDiagonals(array):

    # swap elements of diagonal
    for i in range(N):
        if (i != N / 2):
            temp = array[i][i];
            array[i][i] = array[i][N - i - 1];
            array[i][N - i - 1] = temp;

    for i in range(N):
        for j in range(N):
            print(array[i][j], end = " ");
        print();

# Driver Code
if __name__ == '__main__':
    array = [ 4, 5, 6 ],[ 1, 2, 3 ],[ 7, 8, 9 ];
    interchangeDiagonals(array);

# This code is contributed by Rajput-Ji

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 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!