1572.Matrix Diagonal Sum
常规解法如下，目前没发现有更好的方法，时间复杂度优于O(N).

class Solution {
public:
    int diagonalSum(vector<vector<int>>& mat) {
        int n = mat.size(),sum = 0;
        for(int i = 0;i < n;i++)
        {
            sum += mat[i][i] + mat[i][n -1 -i];    
        }
        
        //sub middle element,when n is odd,it add twice.
        if(n & 1)
            sum = sum - mat[n/2][n/2];

        return sum;       
    }
};