【leetcode】1572. Matrix Diagonal Sum

题目如下：

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

解题思路：我的方法是先累加主对角线，然后再次对角线，考虑到有元素会同时在两个对角线上，所以在累加主对角线的时候，每加完一个元素，将其值置为0。

代码如下：

class Solution(object):
    def diagonalSum(self, mat):
        """
        :type mat: List[List[int]]
        :rtype: int
        """
        res = 0
        for i in range(min(len(mat),len(mat[0]))):
            res += mat[i][i]
            mat[i][i] = 0

        for i in range(len(mat[0])):
            res += mat[len(mat) - i - 1][i]

        return res