Roate a 2D matrix clockwise by 90 degrees

rotateMatrix.py

"""
You are given an n x n 2D matrix that represents an image.
Rotate the image by 90 degrees (clockwise) using O(1) extra space
"""

def rotateImageClockwiseBy90(a):
    """
    The solution here is to find the elements to swap. We will do it
    in cycles. First, identify the groups:
    (x, y), (y, N-1-x), (N-1-x, N-1-y), (N-1-y, x)

    Then, depending on the direction, swap them.
    Here's we'll swap in the clockwise direction
    """
    N = len(a)
    for x in range(N//2):
        for y in range(x, N-x-1):
            t = a[x][y]
            a[x][y] = a[N-1-y][x]
            a[N-1-y][x] = a[N-1-x][N-1-y]
            a[N-1-x][N-1-y] = a[y][N-1-x]
            a[y][N-1-x] = t
    return a


if __name__ == "__main__":
    matrix = [[1, 2, 3],
              [4, 5, 6],
              [7, 8, 9]]
    expected = [[7, 4, 1],
                [8, 5, 2],
                [9, 6, 3]]
    print(rotateImageClockwiseBy90(matrix) == expected)