48. Rotate Image.cpp

/*
You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).
You have to rotate the image in-place, which means you have to modify the input 2D matrix directly. DO NOT allocate another 2D matrix and do the rotation.

Example 1:
Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2:
Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]
 
Constraints:
n == matrix.length == matrix[i].length
1 <= n <= 20
-1000 <= matrix[i][j] <= 1000
*/ 

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        int n = matrix.size();

        //transpose
        for(int i=0; i<n; i++)
        {
            for(int j=0; j<i; j++)
            {
                swap(matrix[i][j], matrix[j][i]);
            }
        }

        // reverse
        for(int k=0; k<n; k++)
        {
            reverse(matrix[k].begin(), matrix[k].end());
        }
    }
};