LeetCode: Rotate Image 解题报告

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

Rotate the image by 90 degrees (clockwise).

Follow up:
Could you do this in-place?

SOLUTION 1:

我们可以把它当作多个嵌套的环来处理，一环加一环。从外环处理到内环。为了方便处理各种下标，我们定义top, bottom, left, right来限制环的左右上下边界。

1. 令tmp = 上边界

2. 上边等于左边元素

3. 左边等于下边元素

4. 下边等于右边元素

5. 右边等于上边元素（tmp）

使用n来记录每一圈的边长。一圈进行4次操作，每次操作移动n - 1个元素即可。

public class Solution {
    public void rotate(int[][] matrix) {
        if (matrix == null || matrix.length == 0 
           || matrix[0].length == 0) {
            return;       
        }
        
        int n = matrix.length;
        int top = 0, down = n - 1, left = 0, right = n - 1;
        
        while (n > 1) {
            for (int i = 0; i < n - 1; i++) {
                int tmp = matrix[top][left + i];
                // 另上边等于左边
                matrix[top][left + i] = matrix[down - i][left];
                
                // 另左边等于下边
                matrix[down - i][left] = matrix[down][right - i];
                
                // 另下边等于右边
                matrix[down][right - i] = matrix[top + i][right];
                
                // 另右边等于上边
                matrix[top + i][right] = tmp;
            }
            top++;
            right--;
            left++;
            down--;
            
            n -= 2;
        }
        
        return;
    }
}

GITHUB:

https://github.com/yuzhangcmu/LeetCode_algorithm/blob/master/array/Rotate.java