48. Rotate Image
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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]]
Idea
class Solution:
def rotate(self, matrix: List[List[int]]) -> None:
"""
Do not return anything, modify matrix in-place instead.
"""
l = 0
r = len(matrix) - 1
while l < r:
for i in range(r-l): # for not only "corners"
t = l
b = r
top_left = matrix[t][l + i]
matrix[t][l + i] = matrix[b - i][l] # top left=bottom left
matrix[b - i][l] = matrix[b][r - i] # bottom left=bottom right
matrix[b][r - i] = matrix[t+i][r] # bottom right=top right
matrix[t + i][r] = top_left # top right=top left
l += 1
r -= 1
Approach 2: Reverse
class Solution:
def rotate(self, matrix: List[List[int]]) -> None:
matrix.reverse()
for i in range(len(matrix)):
for j in range(i + 1, len(matrix)):
matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]
