The challenge

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Note:

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 1:

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

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

Example 2:

Given input matrix =
[
  [ 5, 1, 9,11],
  [ 2, 4, 8,10],
  [13, 3, 6, 7],
  [15,14,12,16]
], 

rotate the input matrix in-place such that it becomes:
[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

The first solution

With this solution, we loop through each column and row and swap the elements, then finally we loop through each row one last time and reverse them.

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def rotate(self, matrix: List[List[int]]) -> None:
    # Transpose the matrix
    for i in range(len(matrix)):
        for j in range(i, len(matrix)):
            matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

    # Reverse each row
    for i in range(len(matrix)):
        matrix[i].reverse()

An alternative solution

With this solution, we start by reversing the list and then loop through each column and row. Within this second loop, we swap the elements.

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def rotate(self, matrix: List[List[int]]) -> None:
    n = len(matrix)
    matrix.reverse()
    for x in range(n):
        for y in range(n-1, x-1, -1):
            matrix[x][y], matrix[y][x] = matrix[y][x], matrix[x][y]