Rotate Image

update 2018-10-09 19:43:44

LeetCode

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]
]

Basic Idea:

先旋转最外面一圈，然后向里面一圈，直到中心。利用一个offset变量从0开始到N/2递增。

• Java Code：

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