题解 | #火眼金睛#

#include <cstdarg>
#include <cstdio>
#include <iostream>
#include <map>
#include <set>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    //map记录ask_id与ans_id集合的映射关系
    int N;
    while(cin>>N){
        std::map<int,std::set<int>> ask2ans;
        std::set<int> badman;
        for(int i=0; i<N; i++){//遍历问题
            int ask_id;
            std::cin>>ask_id;//输入提问者ID
            int num;
            cin>>num;//记录每个问题的回答个数

            std::set<int> valid_ans;//记录有效答题人ID
            for(int j=0; j<num; j++){//遍历答题人ID
                //先排除自问自答，再排除重复
                int id;
                std::cin>>id;
                auto it1 = valid_ans.find(id);
                if(id!=ask_id && it1==valid_ans.end()){
                    valid_ans.insert(id);
                }
            }

            auto it2 = ask2ans.find(ask_id);//看是ask_id是否提问过
            if(it2!=ask2ans.end()){//ask_id提问过,合并ans_id集合
                set<int> temp = it2->second;
                valid_ans.insert(temp.begin(),temp.end());
            }
            ask2ans[ask_id] = valid_ans;

        }

        //寻找（ask,ans）与（ans，ask）的对应关系
        for(auto it1 = ask2ans.begin(); it1!=ask2ans.end(); ++it1){//遍历ask_id的map
            for(auto it2 = it1->second.begin(); it2 != it1->second.end(); ++it2){//遍历ans_id集合
                auto it3 = ask2ans.find(*it2);//找ans_id的map
                if(it3 !=ask2ans.end()){//(ask,ans)找到了(ans,X)
                    if(it3->second.find(it1->first)!=it3->second.end()){//(ask,ans)找到了(ans,ask)
                        badman.insert(it1->first);
                        badman.insert(it3->first);
                    }
                }
            }
        }

        //遍历每个人的问题回答者，超过两个作弊就将其也记为作弊

        for(auto it1 = ask2ans.begin(); it1!=ask2ans.end(); ++it1){
            int count=0;
            for(auto it2 = it1->second.begin(); it2!=it1->second.end(); ++it2){
                auto it3 = badman.find(*it2);
                if(it3!=badman.end()){
                    count ++;
                }
            }
            if(count>1){
                badman.insert(it1->first);
            }
        }

        std::cout<<badman.size()<<endl;
        for(auto it = badman.begin(); it!=badman.end(); ++it){
            cout<<*it<<" ";
        }
        if(badman.size()!=0){
            cout<<endl;
        }
    }

    return 0;
}
// 64 位输出请用 printf("%lld")