[Leetcode] Permutations II

Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].

Solution:

 public class Solution {
     public List<List<Integer>> permuteUnique(int[] num) {
         Arrays.sort(num);
         List<List<Integer>> result=new ArrayList<List<Integer>>();
         List<Integer> al=new ArrayList<Integer>();
         boolean[] flag=new boolean[num.length];
         dfs(num,result,al,0,flag);
         return result;
     }

     private void dfs(int[] num, List<List<Integer>> result, List<Integer> al, int level,boolean[] flag) {
         // TODO Auto-generated method stub
         if(level>num.length)
             return;
         if(level==num.length){
             result.add(new ArrayList<Integer>(al));
             return;
         }
         for(int i=0;i<num.length;++i){
             if(!flag[i]){
                 flag[i]=true;
                 al.add(num[i]);
                 dfs(num, result, al, level+1, flag);
                 al.remove(al.size()-1);
                 flag[i]=false;
                 while((i+1<num.length)&&num[i]==num[i+1])
                     ++i;
             }
         }
     }
 }