48. Rotate Image

1. Description

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.

2. Example

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]]

Example 3:
Input: matrix = [[1]]
Output: [[1]]

Example 4:
Input: matrix = [[1,2],[3,4]]
Output: [[3,1],[4,2]]

3. Constraints

• matrix.length == n
• matrix[i].length == n
• 1 <= n <= 20
• -1000 <= matrix[i][j] <= 1000

4. Solutions

Reverse && Diagonal Reverse

n is the number of nodes in m_matrix
Time complexity: O($n^2$)
Space complexity: O(1)

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        reverse(matrix.begin(), matrix.end());

        for (int i = 0; i < matrix.size(); ++i) {
            for (int j = i; j < matrix[0].size(); ++j) {
                swap(matrix[i][j], matrix[j][i]);
            }
        }
    }   
};