matrix-diagonal-sum.java

/**
* Given a square matrix mat, return the sum of the matrix diagonals.
*
* Only include the sum of all the elements on the primary diagonal and all the elements on the secondary diagonal that are not part of the primary diagonal.
*
* Time Complexity - O(n)
* Space Complexity - O(1)
*/
class Solution {
    public int diagonalSum(int[][] mat) {
        
        int diagonalSum = 0;
        int len = mat.length - 1;
        
        for(int i=0; i<mat.length; i++){
            for(int j=0; j<mat[i].length; j++){
                
                if(i == j){
                    diagonalSum += mat[i][j];
                }
                else if(i == (len-j)){
                    diagonalSum += mat[i][j];
                }
            }
        }
        
        return diagonalSum;
    }
}