1572.go

// Source: https://leetcode.com/problems/matrix-diagonal-sum
// Title: Matrix Diagonal Sum
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// 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:
//
//   https://assets.leetcode.com/uploads/2020/08/14/sample_1911.png
//
//   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
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

package main

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

	sum := 0
	for i := 0; i < n; i++ {
		sum += mat[i][i] + mat[i][n-i-1]
	}
	if n%2 == 1 {
		sum -= mat[n/2][n/2]
	}
	return sum
}