Matrix Diagonal Sum Python Solution with Tests

Tested Python solution for LeetCode 1572 with 19 pytest cases. Generate a practice environment with lcpy.

LeetCode 1572, Easy. Topics: Array, Matrix. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 19 parametrized pytest cases, and a playground notebook:

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

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Problem

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.

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Examples

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

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Constraints

• n == mat.length == mat[i].length
• 1 <= n <= 100
• 1 <= mat[i][j] <= 100

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

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

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Complexity

Time	Space
O(n)	O(1)

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Tags

NeetCode All.