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LeetCode 解题报告(356) -- 48. Rotate Image

Problem

You are given an n x n 2D matrix representing an image, rotate the image by 90 degrees (clockwise).

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:

Example1

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Input: matrix = [[1,2,3],[4,5,6],[7,8,9]]
Output: [[7,4,1],[8,5,2],[9,6,3]]

Example 2:

Example2

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Input: matrix = [[5,1,9,11],[2,4,8,10],[13,3,6,7],[15,14,12,16]]
Output: [[15,13,2,5],[14,3,4,1],[12,6,8,9],[16,7,10,11]]

Example 3:

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Input: matrix = [[1]]
Output: [[1]]

Example 4:

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Input: matrix = [[1,2],[3,4]]
Output: [[3,1],[4,2]]

Constraints:

  • matrix.length == n
  • matrix [i].length == n
  • 1 <= n <= 20
  • -1000 <= matrix [i][j] <= 1000

Analysis

   题目要求把一个矩阵顺时针旋转 90 度,而且不允许使用额外的空间。这就要求我们要进行 in-place 的操作。这里直接提供一个思路,首先按照对角线,把矩阵分成上下两部分,然后交换这两部分,之后再把每一行反转就能得到结果了。


Solution

   无


Code

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class Solution {
public:
void rotate(vector<vector<int>>& matrix) {
int m = matrix.size ();
for (int i = 0; i < m; i++) {
for (int j = i + 1; j < m; j++) {
swap (matrix [i][j], matrix [j][i]);
}
reverse (matrix [i].begin (), matrix [i].end ());
}
}
};

Summary

   这道题目提供了一个在变换矩阵时一个很重要的思路,就是借助对角线。这道题目的分享到这里,感谢你的支持!

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