Rotate Image - Leetcode 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.

The trick here is to realize that cells in our matrix (M) can be swapped out in groups of four in a self-contained manner. This will allow us to keep our space complexity down to O(1).

For each ring, we'll need to perform a number of iterations equal to the length of the side minus 1, since we will have already swapped the far corner as our first iteration. As noticed earlier, the length of the side of a ring is shortened by 2 for each layer of depth we've achieved (len = n - 2 * i - 1).

Inside the nested for loops, we need to perform a four-way swap between the linked cells. In order to save on some processing, we can store the value of the opposite side of i (opp = n - 1 - i) as that value will be reused many times over.

Once the nested loops are finished, M has been successfully transformed in-place.

• Time Complexity: O(N^2) where N is the length of each side of the matrix
• Space Complexity: O(1)