【LeetCode】46. Permutations (2 solutions)

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].

解法一：递归

class Solution {
public:
    vector<vector<int> > permute(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
        {
            for(int i = pos; i < num.size(); i ++)
            {//swap all the ints to the current position
                swap(num[pos], num[i]);
                Helper(ret, num, pos+1);
                swap(num[pos], num[i]);
            }
        }
    }
};

解法二：just a joke

别忘了先排序，因为next_permutation是升序返回的。

class Solution 
{
public:
    vector<vector<int> > permute(vector<int> &num) 
    {
        vector<vector<int> > result;
        //sort first
        //note that next_permutation is in ascending order
        sort(num.begin(), num.end());
        result.push_back(num);
        while(next_permutation(num.begin(), num.end()))
        {
            result.push_back(num);
        }
        return result;
    }
};