[leetcode] 1572. Matrix Diagonal Sum

Description

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

分析

题目的意思是：给你一个矩阵，求出正对角线和斜对角线的值的和，每个位置的值只能计算一次，因此算法很直接，首先遍历求出正斜对角线的和，对于长度为奇数的矩阵的和，需要减去中心位置的值，因为重复加了两次。

代码

class Solution:
    def diagonalSum(self, mat: List[List[int]]) -> int:
        n=len(mat)
        res=0
        for i in range(n):
            res+=mat[i][i]
            res+=mat[i][n-i-1]
        if(n%2==1):
            res-=mat[n//2][n//2]
        return res