1, Boundary Check

/* A utility function to check if x, y is valid
   index for N*N maze */
bool isSafe(int maze[N][N], int x, int y)
{
    // if (x, y outside maze) return false
    if (x >= 0 && x < N && y >= 0 && 
       y < N && maze[x][y] != 0)
        return true;

    return false;
}

2, rotate along the main diagonal and vice diagonal and horizontal line

延主对角线，副对角线，水平线，旋转矩阵

主对角线上的元素是(i,i)，(0, 0), (1, 1), (2, 2).....

主对角线右边的元素 i from 0 to n - 1, j from i+1, to n-1 , 对应主对角线左边的元素matrix[i][j] map matrix[j][i]

副对角线上的元素是 (i, n-1-i), (0,3), (1,2), (2,1), (3,0) ...n=4.

副对角线左边的元素i from 0 to n - 1 j from 0 to n-1-i对应副对角线右边的元素matrix[i][j] map matrix[n-1-j][n-1-i]

for (int i = 0; i < n; ++i) // 沿着主对角线反转
    for (int j = i + 1; j < n; ++j)
        swap(matrix[i][j], matrix[j][i]);

for (int i = 0; i < n; ++i)  // 沿着副对角线反转
        // for (int j = 0; j < n - i; ++j)  // also work
    for (int j = 0; j < n - 1 - i; ++j)
        swap(matrix[i][j], matrix[n - 1 - j][n - 1 - i]);


for (int i = 0; i < n / 2; ++i) // 沿着水平中线反转
    for (int j = 0; j < n; ++j)
        swap(matrix[i][j], matrix[n - 1 - i][j]);

3，cross elements traversal

set matrix zero