作者： 负雪明烛
id： fuxuemingzhu
个人博客： http://fuxuemingzhu.cn/

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题目地址：https://leetcode.com/problems/rotate-image/description/

题目描述

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]

]

题目大意

把矩阵原地顺时针旋转90度。

解题方法

做法挺简单，先上下翻转，再延左上到右下的对角线进行翻转(镜像操作)。

需要注意的是上下翻转的时候是rows-i-1，而不是rows-i。

代码：

class Solution(object):

    def rotate(self, matrix):

        """

        :type matrix: List[List[int]]

        :rtype: void Do not return anything, modify matrix in-place instead.

        """

        if matrix:

            rows = len(matrix)

            cols = len(matrix[0])

            for i in xrange(rows / 2):

                for j in xrange(cols):

                    matrix[i][j], matrix[rows - i - 1][j] = matrix[rows - i - 1][j], matrix[i][j]

            for i in xrange(rows):

                for j in xrange(i):

                    matrix[i][j], matrix[j][i] = matrix[j][i], matrix[i][j]

二刷的时候使用的另外一种旋转的方法：先左右镜像翻转，然后再沿着副对角线（从右上到左下的对角线）进行翻转。比上面的麻烦一点。

使用的C++代码如下：

class Solution {

public:

    void rotate(vector<vector<int>>& matrix) {

        if (!matrix.size()) return;

        const int N = matrix.size();

        for (int i = 0; i < N; i ++) {

            for (int j = 0; j < N / 2; j ++) {

                int tmp = matrix[i][j];

                matrix[i][j] = matrix[i][N - j - 1];

                matrix[i][N - j - 1] = tmp;

            }

        }

        for (int i = 0; i < N; i ++) {

            for (int j = 0; j < N - i; j ++) {

                int tmp = matrix[i][j];

                matrix[i][j] = matrix[N - j - 1][N - i - 1];

                matrix[N - j - 1][N - i - 1] = tmp;

            }

        }

    }

};

日期

2018 年 3 月 5 日
2018 年 12 月 30 日 —— 周赛差强人意