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

Rotate the image by 90 degrees (clockwise).

 

Example

Given a matrix

[    [1,2],    [3,4]]

rotate it by 90 degrees (clockwise), return

[    [3,1],    [4,2]] 分析： 本题的关键是找到几个互换点之间位置的关系。

1 public class Solution { 2     /** 3      * @param matrix: 4      *            A list of lists of integers 5      * @return: Void 6      */ 7     public void rotate(int[][] matrix) { 8         if (matrix == null || matrix.length == 0) return; 9         int n = matrix.length - 1;10         for (int i = 0; i < (n + 1) / 2; ++i) {11             for (int j = i; j < n - i; ++j) {12                 int t = matrix[i][j];13                 matrix[i][j] = matrix[n - j][i];14                 matrix[n - j][i] = matrix[n - i][n - j];15                 matrix[n - i][n - j] = matrix[j][n - i];16                 matrix[j][n - i] = t;17             }18         }19     }20 }

 对于一个正方形边上的四个对应点，它们有下面的关系：n = 边长 - 1

上： (x, y)

下：(n - x, n - y)

左：(n - y, x)

右：(y, n - x)

另一种解法：

The idea was firstly transpose the matrix and then flip it symmetrically. For instance,

1  2  3             4  5 6 7 8 9

after transpose, it will be swap(matrix[i][j], matrix[j][i])

1  4  72  5 8 3 6 9

Then flip the matrix horizontally. (swap(matrix[i][j], matrix[i][matrix.length-1-j])

7  4  18  5 2 9 6 3

Hope this helps.

1 public class Solution { 2     public void rotate(int[][] matrix) { 3         for(int i = 0; i