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:

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

Example 2:

1 2	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:

1 2	Input: matrix = [[1]] Output: [[1]]

Example 4:

1 2	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

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！