[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].

class Solution {
public:
    void DFS(vector<vector<int> > &res,vector<int> &ans,vector<bool> &flag,vector<int> &num)
    {
        if(ans.size()==num.size())
        {
            res.push_back(ans);
            return;
        }
        for(int i=0;i<num.size();i++)
        {
            if(i&&num[i]==num[i-1]&&!flag[i-1]) continue;
            if(!flag[i])
            {
                ans.push_back(num[i]);
                flag[i]=true;
                DFS(res,ans,flag,num);
                flag[i]=false;
                ans.pop_back();
            }
        }
    }
    vector<vector<int> > permuteUnique(vector<int> &num) {
        sort(num.begin(),num.end());//先排序
        vector<vector<int> > res;
        vector<bool> flag(num.size(),false);
        vector<int> ans;
        DFS(res,ans,flag,num);
        return res;
    }
};