[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?

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(matrix.size() == 0) return;
        vector<vector<int>> ans;
        for(int j = 0;j < matrix[0].size();j++)
        {
            vector<int> row;
            for(int i = matrix.size() - 1;i >= 0;i--)
                row.push_back(matrix[i][j]);
            ans.push_back(row);
        }
        matrix = ans;
    }
};