LeetCode - Refresh - Rotate Image

This is a math trick.  

class Solution {
public:
    void rotate(vector<vector<int> > &matrix) {
        auto swap = [](int &a, int &b){int t = a; a = b; b = t;};
        if (matrix.size() == 0) return;
        int n = matrix.size();
        for (int i = 0; i < n; i++) {
            for (int j = i; j < n; j++) {
                swap(matrix[i][j], matrix[j][i]);
            }
        }
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < n/2; j++) {
                swap(matrix[i][j], matrix[i][n-j-1]);
            }
        }
    }
};