Rotate a 2D array without using extra space

Given a N x N 2D matrix Arr representing an image. Rotate the image by 90 degrees (anti-clockwise). You need to do this in place. Note that if you end up using an additional array, you will only receive partial score.

• Time : O(n^2)
• Space : O(1)

class Solution{
    void helper(vector<vector<int>>& arr, int n, int start) {
        if(n <= 1) return;
        int y = start, x = start;
        for(int i = 0; i < n - 1; i++) {
            int tmp = arr[y][x + i];
            arr[y][x + i] = arr[y + i][x + n - 1];
            arr[y + i][x + n - 1] = arr[y + n - 1][x + n - 1 - i];
            arr[y + n - 1][x + n - 1 - i] = arr[y + n - 1 - i][x];
            arr[y + n - 1 - i][x] = tmp;
        }

        helper(arr, n - 2, start + 1);
    }
public:
    void rotateMatrix(vector<vector<int>>& arr, int n) {
        helper(arr, n, 0);
    }
};