【ATT】Combinations

求子集合C(n,k)问题；---DFS

    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > result;
        if(n<=0||k<0||k>n)
            return result;
        vector<int> set;
        subset(0,n,k,set,result);
        return result;
    }
    
    void subset(int curPos,int& n,int& k,vector<int>& set, vector<vector<int> >& result)
    {
        if(set.size()==k)
        {
            result.push_back(set);
            return;
        }
        for(int i=curPos;i<n;i++)
        {
            set.push_back(i+1);
            subset(i+1,n,k,set,result);
            set.pop_back();
        }
    }

　　

另外一种写法：更好理解

vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function

        vector<int> temp;
        vector<vector<int> > ans;
        combine_rec(ans , temp , n , k , 1);
        return ans;
    }

    void combine_rec(vector<vector<int> > &ans , vector<int> temp , int n , int k , int index) {

        if (k == 0)
            ans.push_back(temp);

        else if (k > 0 && index <= n)
        {
            temp.push_back(index);    //选择了index
            combine_rec(ans , temp , n , k-1 , index+1);  //注意k-1

            temp.pop_back();
            combine_rec(ans , temp , n , k , index+1);   //不选择index，注意是k
        }
    }