README.md

Rotate Image

Description

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]
]

思路

题意是给一个 n x n 的二维数组，将数组元素顺时针旋转 90 度。需要直接修改当前数组。

思路是从外往内，一圈一圈的进行。其中每一圈的每个元素顺时针旋转 90 度，连续操作四次。注意边界，防止重复操作。

实现代码

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