Software Engineering

# Rotate a Matrix List in Python

## 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.

``````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.

``````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]
``````