leetcode Combinations

此题利用递归回溯思想

代码

class Solution {public:    vector<vector<int> > combine(int n, int k) {        vector<vector<int> > result;        if(n<k||k==0)            return result;                    vector<int> temp;        combineHelper(result, temp, n, k, 1);                return result;            }            void combineHelper(vector<vector<int> > &result, vector<int> temp, int n, int k, int start)    {        if(temp.size()==k)        {            result.push_back(temp);            return ;        }        for(int i = start; i <= n; ++i)        {        temp.push_back(i);        combineHelper(result, temp, n, k, i+1);        temp.pop_back();        }    }};