题目地址：https://leetcode.com/problems/rotate-image/description/
题目描述
Youare 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]
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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;
}
}
}
};
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