【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 ;
    }
    
};