48. Rotate Image

matrix가 주어지면 space O(1)으로 90도 시계방향으로 회전한다

/*
     * clockwise rotate
     * first reverse up to down, then swap the symmetry
     * 1 2 3     7 8 9     7 4 1
     * 4 5 6  => 4 5 6  => 8 5 2
     * 7 8 9     1 2 3     9 6 3
*/
vector<vector<int>> solution(vector<vector<int>> a) {
    int n = a.size();
    int m = a[0].size();
    
    // reverse top down
    for (int i = 0; i < n / 2; i++) {
        for(int j = 0; j < m; j++)
            swap(a[i][j], a[a.size() - 1 - i][j]);
    }
    
    // swap the symmetry
    for (int i = 0; i < n; i++) {
        for (int j = 0; j < i; j++) {
            swap(a[i][j], a[j][i]);
        }
    }
    
    return a;
}