LeetCode 048 Rotate Image

LeetCode

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

题目大意：

顺时针循环矩阵90度

解题思路：

先上下对称，再沿正对角线(左上到右下)对称。正对角线实现比较容易

解题步骤：

N/A

注意事项：

先上下对称，再沿正对角线(左上到右下)对称。正对角线实现比较容易

Python代码：

def rotate(self, matrix: List[List[int]]) -> None:
	for i in range(len(matrix) // 2):
		for j in range(len(matrix[0])):
			matrix[i][j], matrix[len(matrix) - 1 - i][j] = matrix[len(matrix) - 1 - i][j], matrix[i][j]

	for i in range(len(matrix)):
		for j in range(i, len(matrix[0])):
			matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

算法分析：

时间复杂度为O(n²)，空间复杂度O(1)