LeetCode，leetcodeoj

LeetCode，leetcodeoj

題目：

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想象成四塊，把第一塊摳出來並copy一份，然後旋轉式copy，代碼較長。

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) 其實可以將上面的代碼進行簡化合并，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);    }}