77. Combinations

class Solution {public:    vector<int> s;    int maxLevel, maxNum;    vector<vector<int>> ans;    vector<vector<int>> combine(int n, int k) {        if(k){            maxLevel = k; maxNum = n;            for(int i = 1; i <= maxNum; ++i)                dfs(i, 1);        }        return ans;    }    void dfs(int start, int level){        if(start > maxNum)            return;        else            s.push_back(start);        if(level == maxLevel){            ans.push_back(vector<int> (s));        }else{            for(int i = start+1; i <= maxNum; ++i)                dfs(i, level + 1);        }        s.pop_back();    }};