[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;
            }
        }    
    }
}