【LeetCode】Permutations

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

code: STL method and recursion method.

 

class Solution {

public:

    vector<vector<int> > permute(vector<int> &num) {

        // Note: The Solution object is instantiated only once and is reused by each test case.

        vector<vector<int> > res;

        if(num.size() == 0) 

            return res;

        //vector<int> tmp;

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

        res.push_back(num);

        while( next_permutation(num.begin(),num.end()) )

        {

            res.push_back(num);

        }

        return res;

    }

};

recursion vision:

 

 

class Solution {

public:

    

    vector<vector<int> > permute(vector<int> &num) {

        // Note: The Solution object is instantiated only once and is reused by each test case.

        vector<vector<int> > res;

        if(num.size() == 0) 

            return res;

        vector<int> tmp;

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

        map<int,int> hash;

        permutation(0,num,res,hash,tmp);

        return res;

        

    }

    void permutation(int pos, vector<int> &num, vector<vector<int> > &res, map<int,int> &hash, vector<int> &tmp)

    {

        if(pos == num.size())

        {

            res.push_back(tmp);

            return ;

        }

        for(int i = 0; i < num.size(); i++)

        {

            if(hash[num[i]] != 1)

            {

                tmp.push_back(num[i]);

                hash[num[i]] = 1;

                permutation(pos + 1, num, res, hash, tmp);

                hash[num[i]] = 0;

                tmp.pop_back();

            }

        }

        return ;

    }

    

};