Rotate Image

Question:

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?

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可以先旋转最外围一层，然后依次旋转里层：

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        int N=matrix.size();
    for(int layer=0;layer<N/2;layer++)
    {
        int first=layer;
        int last=N-1-layer;
        for(int i=first;i<last;i++)
        {
            int offset=i-first;
            int top=matrix.at(first).at(i);  //save top
            matrix.at(first).at(i)=matrix.at(last-offset).at(first);  //left->top
            matrix.at(last-offset).at(first)=matrix.at(last).at(last-offset); //bottom->left
            matrix.at(last).at(last-offset)=matrix.at(i).at(last);  //right->bottom
            matrix.at(i).at(last)=top;  //top->right
        }
    }
        
    }
};