【题解】【矩阵】【回溯】【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?

思路：

每次转着圈挪四个元素，这步只需要一个int的额外空间，想象一个方阵

哦不好意思，这个太不专业了，咱换个大一点6×6的

草图上比划比划第二层的元素怎么移动，一次Accept的感觉好爽

代码：

void rotate(vector<vector<int> > &matrix) {
    int n = matrix.size();
    if(n < 2) return;
    
    for(int layer = 0; layer < n/2; layer++){
        for(int i = layer; i < n-1-layer; i++){//避免重复处理n-1-layer
            int leftTop = matrix[layer][i];//注意二维矩阵的(i,j)index与数学坐标(x,y)中是相反的
            matrix[layer][i] = matrix[n-1-i][layer]; //先在纸上想清楚赋值的先后循序
            matrix[n-1-i][layer] = matrix[n-1-layer][n-1-i];
            matrix[n-1-layer][n-1-i] = matrix[i][n-1-layer];
            matrix[i][n-1-layer] = leftTop;
        }
    }
}