[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].

以下是我AC的代码，使用了递归和回溯的方式：

/**

 * author: Zhou jianxin

 */



class Solution

{

public:

    vector<vector<int>> permuteUnique(vector<int> &num) 

    {

       vector<vector<int>> ret;

       vector<int> list;

       vector<bool> flag(num.size(), false);

       sort(num.begin(), num.end());

       permuteUniqueRec(num, list, flag, ret);

       return ret;

    }



private:

    void permuteUniqueRec(const vector<int> &num, 

                          vector<int> &list, 

                          vector<bool> &flag,

                          vector<vector<int>> &ret)

    {

        if (list.size() == num.size())

        {

            ret.push_back(list);

            return;

        }

        

        for (size_t ix = 0; ix != num.size(); ++ix)

        {

            if (flag[ix] || (ix != 0 && !flag[ix - 1] && num[ix - 1] == num[ix]))

            {

                continue;

            }

            

            list.push_back(num[ix]);

            flag[ix] = true;

            

            permuteUniqueRec(num, list, flag, ret);

            

            list.erase(list.end() - 1);

            flag[ix] = false;

        }

    }

};

By the way，今天是平安夜，刷题来过，倒也是我生平第一次，哈哈。祝大家节日快乐。