1572. 矩阵对角线元素的和

class Solution {
    public int diagonalSum(int[][] mat) {
        int sum = 0, len = mat[0].length;
        for (int i = 0; i < len; i++){
            sum += mat[i][i];
        }
        for (int i = 0; i < len; i++){
            for (int j = 0; j < len; j++){
                if (i + j == len - 1)
                    sum += mat[i][j];
            }
        }
        return len%2 == 1 ? sum - mat[(len - 1)/2][(len - 1)/2] : sum;
    }
}