Permutations @LeetCode

 
package Level3;  
  
import java.util.ArrayList;  
  
/** 
 * Permutations 
 *  
 *  Given a collection of numbers, return all possible permutations. 
 
For example, 
[1,2,3] have the following permutations: 
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1]. 
 * 
 */  
public class S46 {  
  
    public static void main(String[] args) {  
        int[] S = {1,2,3};  
        System.out.println(permute(S));  
    }  
  
    public static ArrayList> permute(int[] S) {  
        ArrayList> ret = new ArrayList>();  
        ArrayList list = new ArrayList();  
        rec(S, ret, list);  
        return ret;  
    }  
      
    public static void rec(int[] S, ArrayList





> ret, ArrayList list){  
        // 当数组长度为0时，添加入ret  
        if(S.length == 0){  
            ret.add(new ArrayList(list));        // 必须基于现场新建一个ArrayList然后添加入ret！  
            return;  
        }  
          
        // 遍历数组中的每一个数作为第一个元素  
        for(int i=0; i