#48 Rotate Image——Top 100 Liked Questions-蒲公英云

#48 Rotate Image——Top 100 Liked Questions

Bertha 。 2022-01-30 03:15 293阅读 0赞

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]
]

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第一次：通过找规律，发现(i, j)—>(j, n-1-i)，但是直接交换是有错的

第二次：网上参考：先以对角线为轴翻转，再以x轴中线翻转，蓝色数字是翻转轴，可以通过找规律发现i和j的范围以及交换的zuobao（https://www.cnblogs.com/grandyang/p/4389572.html）

“””
class Solution(object):
    def rotate(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: None Do not return anything, modify matrix in-place instead.
        """
        n = len(matrix)
        for i in range(n):
            for j in range(n - i - 1):
                tmp = matrix[i][j]
                matrix[i][j] = matrix[n - 1 - j][n - 1 - i]
                matrix[n - 1 - j][n - 1 - i] = tmp
        for i in range(n // 2):
            for j in range(n):
                tmp = matrix[i][j]
                matrix[i][j] = matrix[n - 1 - i][j]
                matrix[n - 1 - i][j] = tmp
“””

Runtime: 20 ms, faster than 98.51% of Python online submissions for Rotate Image.

Memory Usage: 11.8 MB, less than 30.58% of Python online submissions for Rotate Image.

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