【leetcode刷题笔记】Rotate Image

You are given an n x n 2D matrix representing an image.

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

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题解：

如果要原地算法的话，就只能用交换实现了。首先把matrix[i][j]和matrix[j][i]交换，然后把整个矩阵左右翻转，如下图所示：

1 2 3                   1 4 7                      7 4 1

4 5 6                   2 5 8                      8 5 2

7 8 9                   3 6 9                      9 6 3

JAVA代码：

public class Solution {
    public void rotate(int[][] matrix) {
        int n = matrix.length;
        for(int i = 0;i < n;i++){
            for(int j = i+1;j < n;j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[j][i];
                matrix[j][i] = temp;
            }
            for(int j = 0;j < n/2;j++){
                int temp = matrix[i][j];
                matrix[i][j] = matrix[i][n-j-1];
                matrix[i][n-j-1] = temp;
            }
        }
    }
}