LeetCode

题目：

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?

思路：

1) 把matrix想象成四块，把第一块抠出来并copy一份，然后旋转式copy，代码较长。

package rotation;

public class RotateImage {

    public void rotate(int[][] matrix) {
        int n = matrix.length;
        int rows = n / 2;
        int cols = n % 2 == 0 ? rows : (rows + 1);
        int[][] tmp = new int[rows][cols];
        for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                tmp[i][j] = matrix[i][j];
            }
        }
        
        for (int i = rows; i < n; ++i) {
            for (int j = 0; j < cols; ++j) {
                matrix[j][n - 1 - i] = matrix[i][j];
            }
        }
        
        for (int i = cols; i < n; ++i) {
            for (int j = rows; j < n; ++j) {
                matrix[j][n - 1 - i] = matrix[i][j];
            }
        }
        
        for (int i = 0; i < cols; ++i) {
            for (int j = cols; j < n; ++j) {
                matrix[j][n - 1 - i] = matrix[i][j];
            }
        }
        
        for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                matrix[j][n - 1 - i] = tmp[i][j];
            }
        }
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        // int[][] matrix = {{1,2,3},{5,6,7},{9,10,11}};
        int[][] matrix = {{1,2},{3,4}};
        RotateImage r = new RotateImage();
        r.rotate(matrix);
    }

}

2) 其实可以将上面的代码进行简化合并，如下

package rotation;

public class RotateImage {

    public void rotate(int[][] matrix) {
        int n = matrix.length;
        int rows = n / 2;
        int cols = n - rows;
        for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                int tmp = 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] = tmp;
            }
        }
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        // int[][] matrix = {{1,2,3},{5,6,7},{9,10,11}};
        int[][] matrix = {{1,2},{3,4}};
        RotateImage r = new RotateImage();
        r.rotate(matrix);
    }

}