Leetcode 77. 组合 (排列组合，回溯）

输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

class Solution {
public:
    vector<vector<int>>ans;
    vector<vector<int>> combine(int n, int k) {
        vector<int>tep;
        dfs(n,0,tep,k,1);
        return ans;
    }
    inline void dfs(int n,int now,vector<int>tep,int k,int index)
    {
        if(now==k)
        {
            ans.push_back(tep);
            return ;
        }
        for(int i=index;i<=n;i++)
        {
            tep.push_back(i);
            dfs(n,now+1,tep,k,i+1);
            tep.pop_back();
        }
    }
};