学步园

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].

思路：假设有集合[1,2,3]，来一个4,则有集合[4,1,2,3]、[1,4,2,3]、[1,2,4,3]、[1,2,3,4]。故code如下：

class Solution {
public:
    vector<vector<int> > permute(vector<int> &num) {
        vector<vector<int> > aa,tmp;
        vector<int> a;
        aa.clear();
        if(num.empty())
        {
            return aa;
        } 
        int len = num.size();
        int i,j,k,m,co,t;
        for(i=0; i<len; ++i)
        {
            a.clear();
            if (aa.empty())
            {
                a.push_back(num[i]);
                aa.push_back(a);
                continue;
            }   
            tmp.clear();
            co = aa.size();
            
            for(j=0; j<co; ++j)
            {
                m = aa[j].size();
                for(k=0; k<=m; ++k)
                {
                    a.clear();
                    for(t=0; t<k; ++t)
                    {
                        a.push_back(aa[j][t]);
                    } 
                    a.push_back(num[i]);
                    for( ; t<m; ++t)
                    {
                        a.push_back(aa[j][t]);
                    }       
                    tmp.push_back(a);
                }    
            }     
            aa = tmp;
        }      
        return aa;             
    }
};