[LeetCode] Rotate Image

题目：

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

Rotate the image by 90 degrees (clockwise).

Follow up: Could you do this in-place?

分析：

要达到in-place的要求，我们采用交换的方法，顺序是左上先和左下交换，然后左上和右下交换，然后左上和右上交换。

代码：

public class RotateImage {public void rotate(int[][] matrix) {//计算圈数int n = matrix.length, lvl = n / 2;for(int i = 0; i < lvl; i++) {for(int j = i; j < n - i - 1; j++) {//左上和左下交换swap(matrix, i, j, j, n - i - 1);//左上和右下交换swap(matrix, i, j, n - i - 1, n - j - 1);//左上和右上交换swap(matrix, i, j, n - j - 1, i);}}}private void swap(int[][] matrix, int i1, int j1, int i2, int j2) {int tmp = matrix[i1][j1];matrix[i1][j1] = matrix[i2][j2];matrix[i2][j2] = tmp;}}