Link to Question

MEDIUM

Rotate Image

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

Input:

matrix = [[1,2,3],[4,5,6],[7,8,9]]

Output:

[[7,4,1],[8,5,2],[9,6,3]]

Explanation:
Rotate the matrix by 90 degrees clockwise.

---

Constraints

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

---

Solution: In-Place Rotation

• Time Complexity: O(n^2)
• Space Complexity: O(1)

C++

class Solution {
public:
    void rotate(vector<vector<int>>& m) {
        int n = m.size();
        for (int i = 0; i < n; i++)
            for (int j = i; j < n; j++)
                swap(m[i][j], m[j][i]);    // Transpose
        for (int i = 0; i < n; i++)
            for (int j = 0; j < n/2; j++)
                swap(m[i][j], m[i][n-j-1]); // Reverse each row
    }
};