48. Rotate Image（旋转矩阵）

 

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

Rotate the image by 90 degrees (clockwise).

Note:
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:

Given input matrix =
[
  [1,2,3],
  [4,5,6],
  [7,8,9]
],

rotate the input matrix in-place such that it becomes:
[
  [7,4,1],
  [8,5,2],
  [9,6,3]
]

Example 2:

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

rotate the input matrix in-place such that it becomes:
[
  [15,13, 2, 5],
  [14, 3, 4, 1],
  [12, 6, 8, 9],
  [16, 7,10,11]
]

先上下翻转，然后在对称翻转。

/*
 * clockwise rotate
 * first reverse up to down, then swap the symmetry
 * 1 2 3     7 8 9     7 4 1
 * 4 5 6  => 4 5 6  => 8 5 2
 * 7 8 9     1 2 3     9 6 3
*/

 class Solution {

      public void rotate(int[][] matrix) {
             int rows = matrix.length - 1;
             int cols = matrix[0].length - 1;
             for(int i = 0;i <=rows/2;i++)
                 for(int j = 0;j <= cols;j++ )
                 swap2(matrix,i,j,cols-i,j);

             for(int i = 0;i<=rows;i++)
                 for(int j =i+1;j<=cols;j++)
                     swap2(matrix, i, j,j,i);
         }

         private void swap2(int[][] a,int i1,int j1,int i2,int j2) {
             int temp = a[i1][j1];
             a[i1][j1] = a[i2][j2];
             a[i2][j2] = temp;

         }

 }