[LeetCode] Rotate Image

Problem: 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]]

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]]

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

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

Approach: The approach is straight forward:

• Transpose the matrix.
• Reverse each row. 

Implementation in C#:

    public void Rotate(int[][] matrix)

    {

        int n = matrix?.Length ?? 0;

        if (n == 0)

        {

            return;

        }

        for (int i = 0; i < n; ++i)

        {

            for (int j = 0; j <= i; ++j)

            {

                int temp = matrix[i][j];

                matrix[i][j] = matrix[j][i];

                matrix[j][i] = temp;

            }

        }

        for (int i = 0; i < n; ++i)

        {

            Array.Reverse(matrix[i]);

        }

    }

Complexity: O(n^2)