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1572. Matrix Diagonal Sum

Difficulty Topics

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

Solution

matrix-diagonal-sum.py
class Solution:
    def diagonalSum(self, mat: List[List[int]]) -> int:
        rows, cols = len(mat), len(mat[0])
        res = 0

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

        if rows & 1:
            res -= mat[rows // 2][rows // 2]

        return res
matrix-diagonal-sum.cpp
class Solution {
public:
    int diagonalSum(vector<vector<int>>& mat) {
        int res = 0;
        int N = mat.size();

        for (int i = 0, j = N-1; i < N; i++, j--){
            if (i == j){
                res += mat[i][i];
            }else{
                res += mat[i][j] + mat[i][i];
            }
        }

        return res;
    }
};