77题

方法：

class Solution {
public:
    vector<vector<int>> results;
    vector<int> result;
    vector<vector<int>> combine(int n, int k) {
        dfs(n,k,1);
        return results;
    }
    void dfs(int n, int k,int id){
        if(k==0){
            results.emplace_back(result);
            return;
        }
        for(int i=id;i<=n;i++){
            result.emplace_back(i);
            dfs(n,k-1,i+1);
            result.pop_back();
        }
    }
};