# Rotate Image

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 1:

```Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]
```

Example 2:

```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]]
```

Constraints:

• `n == matrix.length == matrix[i].length`
• `1 <= n <= 20`
• `-1000 <= matrix[i][j] <= 1000`

### Solution :

```class Solution:
def rotate(self, matrix: List[List[int]]) -> None:
"""
Do not return anything, modify matrix in-place instead.
"""
l, r = 0, len(matrix) - 1
while l < r:
for i in range(r - l):
top, bottom = l, r

# Save TopLeft value
topleft = matrix[top][l + i]
# Move bottom left to top left
matrix[top][l + i] = matrix[bottom - i][l]
# move bottom right to bottom left
matrix[bottom - i][l] =  matrix[bottom][r - i]
# move top right to bottom right
matrix[bottom][r - i] = matrix[top + i][r]
# update Top left with variable
matrix[top + i][r] = topleft
l += 1
r -= 1
```

Ashwini Verma on Jan 31, 2022 at 11:01 am

×