Matrix diagonal sum

We will traverse the matrix row by row. For each row, we have to take 2 diagonal position and add it to the result. If the matrix has an odd number of rows, we have to subtract the middle element from the result. Finally, we will return the result.

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

Time complexity: O(n)
Space complexity: O(1)