Leetcode 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?

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解题思路：

将每个[i, j]对应旋转对称的四个点的坐标算出来，然后pixel by pixel的旋转。

公式：matrix[i][j] = matrix[n-1-j][i]   i 表示从左到右，j 表示从右到左。

注意当n是奇数时，比如5，要转换到第3列/行，因此 j < Math.ceil((double) n / 2)

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Java code:

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

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Reference:

1. http://www.programcreek.com/2013/01/leetcode-rotate-image-java/

2. http://www.cnblogs.com/springfor/p/3886487.html