Matrix Diagonal Sum
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.
Example 1:
Input: mat = [[1,2,3],
[4,5,6],
[7,8,9]]
Output: 25
Explanation: Diagonals sum: 1 + 5 + 9 + 3 + 7 = 25
Notice that element mat[1][1] = 5 is counted only once.
Example 2:
Input: mat = [[1,1,1,1],
[1,1,1,1],
[1,1,1,1],
[1,1,1,1]]
Output: 8
Example 3:
Input: mat = [[5]]
Output: 5
Constraints:
n == mat.length == mat[i].length
1 <= n <= 100
1 <= mat[i][j] <= 100
思路一：分别计算两个斜对角，如果矩阵长度 n 时奇数，需要减去重复计算的矩阵中间值
public int diagonalSum(int[][] mat) {
int n = mat.length;
int sum = 0;
for (int i = 0; i < n; i++) {
sum += mat[i][i];
}
for (int i = 0, j = n - 1; i < n && j >= 0; i++, j--) {
sum += mat[i][j];
}
if ((n & 1) == 1) {
sum -= mat[n / 2][n / 2];
}
return sum;
}