leetcode@ [48] Rotate Image

https://leetcode.com/problems/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?

 

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        int n = matrix.size();
        if(n == 0) return;
        
        for(int layer = 0; layer < n/2; ++layer) {
            int first = layer, last = n-1-layer;
            
            for(int i = first; i < last; ++i) {
                int offset = i - first;
                
                //save the top.
                int top = matrix[first][i];
                
                //left -> top.
                matrix[first][i] = matrix[last-offset][first];
                
                //buttom -> left.
                matrix[last-offset][first] = matrix[last][last-offset];
                
                //right -> buttom.
                matrix[last][last-offset] = matrix[i][last];
                
                //top -> right.
                matrix[i][last] = top;
            }
        }
    }
};