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]. 
 

Output: 

 6 5 4
 1 2 3
 9 8 7

Time Complexity: O(N*N), as we are using nested loops for traversing the 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!