Matrix Diagonal Sum Python Solution with Tests

lcpy gen -n 1572   # by problem number
lcpy gen -s matrix_diagonal_sum   # by problem name

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.

Input: mat = [[1,1,1,1],
              [1,1,1,1],
              [1,1,1,1],
              [1,1,1,1]]
Output: 8

Input: mat = [[5]]
Output: 5

class Solution:
    # Time: O(n)
    # Space: O(1)
    def diagonal_sum(self, mat: list[list[int]]) -> int:
        total = 0
        n = len(mat)
        for i in range(n):
            total += mat[i][i]
            if i != n - 1 - i:
                total += mat[i][n - 1 - i]
        return total