Matrix diagonal sum

Array Matrix

84.2% acceptance

Feb 25, 2026

3791

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.

Solution

Rust

Time O(n)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn diagonal_sum(mat: Vec<Vec<i32>>) -> i32 {
    let n = mat.len();
    let mut sum = 0;
    for i in 0..n {
      sum += mat[i][i]; // primary diagonal
      sum += mat[i][n - 1 - i]; // secondary diagonal
    }
    // If n is odd, center element was counted twice
    if n % 2 == 1 {
      sum -= mat[n / 2][n / 2];
    }
    sum
  }
}