LeetCode 48. Rotate Image (Medium)

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本文介绍了一种不使用额外空间直接在原地旋转n*n矩阵的方法。通过两步操作实现：首先沿副对角线翻转矩阵，然后沿水平中线进行翻转。这种方法可以有效地完成题目要求的顺时针旋转90度。

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.

Example1：

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]
]

Example2：

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：给一个n*n矩阵，将它旋转90度。

思路：思路参考了网上的博客，先沿副对角线翻转，再沿水平中线翻转。

c++代码：

class Solution {
public:
    void rotate(vector<vector<int>>& matrix) {
        int size = matrix.size();
        for (int i = 0; i < size; i++)
        {
            for (int j = 0; j < size - i; j++)
            {
                int temp = matrix[i][j];
                matrix[i][j] = matrix[size - 1 - j][size - 1 - i];
                matrix[size - 1 - j][size - 1 - i] = temp;
            }
        }
        for (int i = 0; i < size / 2; i++)
        {
            for (int j = 0; j < size; j++)
            {
                int temp = matrix[i][j];
                matrix[i][j] = matrix[size - 1 - i][j];
                matrix[size - 1 - i][j] = temp;
            }
        }
    }
};