48. Rotate Image

Question:

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]
]

Thinking:

• 这道题主要还是考察对数组的理解。

class Solution {
    public void rotate(int[][] matrix) {
        if(matrix == null || matrix[0].length == 0) return;
        int len = matrix[0].length;
        int depth = len / 2;
        for(int i = 0; i <= depth; i++){
            int l = len - 2 * i - 1;
            for(int j = 0; j < l; j++){
                int temp = matrix[i + j][i];
                matrix[i + j][i] = matrix[len - 1 - i][j + i];    //d -> l
                matrix[len - 1 - i][j + i] = matrix[len - 1 - i - j][len - i - 1];    //r -> d
                matrix[len - 1 - i - j][len - i - 1] = matrix[i][len - 1 - i - j];    //u -> r
                matrix[i][len - 1 - i - j] = temp;
            }
        }
    }
}

二刷

这道题实际上是第三次写，实际上还是找规律题，到底如何定义是要如何定位矩阵中的某一个位置。

class Solution {
    public void rotate(int[][] matrix) {
        if(matrix == null || matrix.length == 0) return;
        int depth = matrix.length / 2;
        int len = matrix.length;
        for(int d = 0; d < depth; d++){
            int l = len - 2 * d - 1;
            for(int j = 0; j < l; j++){
                int temp = matrix[d + j][d];
                matrix[d + j][d] = matrix[len - d - 1][j + d];
                matrix[len - d - 1][j + d] = matrix[len - 1 - j - d][len - d - 1];
                matrix[len - 1 - j - d][len - d - 1] = matrix[d][len - 1 - d- j];
                matrix[d][len - 1 - d- j] = temp;
            }
        }
    }
}

Amazon session

class Solution {
    public void rotate(int[][] matrix) {
        int len = matrix.length;
        if(len <= 1) return;
        for(int d = 0; d < len / 2; d++){
            int step = len - 2 * d - 1; // number of steps to go
            for(int j = 0; j < step; j++){
                int temp = matrix[d][d + j];
                matrix[d][d + j] = matrix[len - d - j - 1][d];
                matrix[len - d - j - 1][d] = matrix[len - d - 1][len - d - j - 1];
                matrix[len - d - 1][len - d - j - 1] = matrix[d + j][len - d - 1];
                matrix[d + j][len - d - 1] = temp;
            }
        }
    }
}