Leetcode: Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

抠细节的题，题目思想如下：to implement the rotation in layers, if size is n, so there will be (int)(n / 2) layers that should be rotated. 

需要注意的是：在下面这段code里面，需要注意的是：每一层layer中 i 的取值范围是 0<= i < m - layer - 1，注意上限是m-layer-1, 不是m-layer，如果是m-layer，就会多移。。造成意想不到的结果

public class Solution {
    public void rotate(int[][] matrix) {
        int m = matrix.length;
        if (m == 0) return; 
        for (int layer = 0; layer < m / 2; layer++) {
            for (int i = layer; i < m - layer - 1; i++) {
                int temp = matrix[layer][i]; //store up elements
                matrix[layer][i] = matrix[m-1-i][layer]; // up is substituted by left
                matrix[m-1-i][layer] = matrix[m-1-layer][m-1-i]; // left is substituted by down
                matrix[m-1-layer][m-1-i] = matrix[i][m-1-layer]; // down is substitued by right
                matrix[i][m-1-layer] = temp; // right is substituted by stored elements
            }
        }
    }
}

Another solution from Leetcode discussion

/*
 * clockwise rotate
 * first reverse up to down, then swap the symmetry 
 * 1 2 3     7 8 9     7 4 1
 * 4 5 6  => 4 5 6  => 8 5 2
 * 7 8 9     1 2 3     9 6 3
*/
void rotate(vector<vector<int> > &matrix) {
    reverse(matrix.begin(), matrix.end());
    for (int i = 0; i < matrix.size(); ++i) {
        for (int j = i + 1; j < matrix[i].size(); ++j)
            swap(matrix[i][j], matrix[j][i]);
    }
}