Pat 1121 damn single (25 points) set

#include <iostream>
#include <vector>
#include <set>
#include <map>
using namespace std;
int main() {
    
    int n,m,temp,cnt=0;
    cin>>n;
    map<int,int>mapp;
    for(int i=0;i<n;i++){
    
        int a,b;
        cin>>a>>b;
        mapp[a]=b;
        mapp[b]=a;
    }
    cin>>m;
    set<int>s,t;
    vector<int> guest(m);
    for(int i=0;i<m;i++){
    
        scanf("%d",&guest[i]);
        s.insert(guest[i]);
    }
    for(int i=0;i<m;i++){
    
        if(s.find(mapp[guest[i]])==s.end())   // It's a single dog 
            t.insert(guest[i]);
    cout<<t.size()<<endl;
    for(auto it=t.begin();it!=t.end();it++){
    
        if(it==t.begin())printf("%05d",*it);
        else printf(" %05d",*it);
    }
    return 0;
}