leetcode——47——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:
    vector<vector<int> > permuteUnique(vector<int> &num) {
        vector<vector<int> > ret;
        Helper(ret, num, 0);
        return ret;
    }
    void Helper(vector<vector<int> >& ret, vector<int> num, int pos)
    {
        if(pos == num.size()-1)
            ret.push_back(num);
        else
        {
            sort(num.begin()+pos, num.end());
            for(int i = pos; i < num.size(); i ++)
            {
                if(i != pos && num[i] == num[i-1])
                    continue;
                swap(num[pos], num[i]);
                Helper(ret, num, pos+1);
                swap(num[pos], num[i]);
            }
        }
    }
};