[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;

    }

};