48. Rotate Image

无名_1989

于 2018-04-23 08:58:26 发布

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本文介绍了一个高效的算法，用于将一个 n x n 的二维矩阵按顺时针方向旋转90度。该算法通过先转置矩阵再水平翻转的方式实现原地旋转，避免了额外的空间开销。

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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]
]

void rotate(vector<vector<int>>& m) {
        int n = m.size();
        
        for(int i=0; i<n; i++)
            for(int j=0; j<i; j++)
                swap(m[i][j], m[j][i]);
        
        for(int i=0; i<n; i++)
            reverse(m[i].begin(), m[i].end());
    }