Brooke

一、題目

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:

 

 

 

 

 

 

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

 

二、程式作法

public class Solution
{
    public int DiagonalSum(int[][] mat)
    {
        int sum = 0;
        for (int i = 0; i < mat.Length; i++)
            for (int j = 0; j < mat[i].Length; j++)
                if (i == j || j == mat[i].Length - 1 - i) sum += mat[i][j];

        return sum;
    }
}

 

三、思路筆記

X 軸由左向右掃，Y 軸由下往上掃，

每次 X 軸移動時，就去對 Y 軸的映像點做相加，直到 X 軸掃完。