48. Rotate Image

Updated: 2023-03-04

LeetCode problem

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]