1572. Matrix Diagonal Sum

🟩 Easy

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

matrix

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

My Solution

func diagonalSum(mat [][]int) int {
    sum := 0

    l, r := 0, len(mat)-1
    for _, nums := range mat {
        if l == r {
            sum += nums[l]
        } else {
            sum += nums[l] + nums[r]
        }

        l++
        r--
    }

    return sum
}

Optimal solution

func diagonalSum(mat [][]int) int {
    n := len(mat)
    sum := 0

    for i := 0; i < n; i++ {
        sum += mat[i][i]                   // Primary diagonal
        sum += mat[i][n-i-1]               // Secondary diagonal
    }

    if n%2 == 1 {
        sum -= mat[n/2][n/2]               // Subtract the middle element if odd-sized matrix
    }

    return sum
}

result

Leetcode: link