PAT 1121 Damn Single

"Damn Single (单身狗)" is the Chinese nickname for someone who is being single. You are supposed to find those who are alone in a big party, so they can be taken care of.

Input Specification:
Each input file contains one test case. For each case, the first line gives a positive integer N (≤ 50,000), the total number of couples. Then N lines of the couples follow, each gives a couple of ID's which are 5-digit numbers (i.e. from 00000 to 99999). After the list of couples, there is a positive integer M (≤ 10,000) followed by M ID's of the party guests. The numbers are separated by spaces. It is guaranteed that nobody is having bigamous marriage (重婚) or dangling with more than one companion.

Output Specification:
First print in a line the total number of lonely guests. Then in the next line, print their ID's in increasing order. The numbers must be separated by exactly 1 space, and there must be no extra space at the end of the line.

Sample Input:
3
11111 22222
33333 44444
55555 66666
7
55555 44444 10000 88888 22222 11111 23333

Sample Output:
5
10000 23333 44444 55555 88888

#include<iostream> //水题
#include<vector>
#include<algorithm>
using namespace std;
int main(){
  vector<int> couples(100000, 0), visited(100000, 0), single, ans;
  int n;
  cin>>n;
  for(int i=0; i<n; i++){
    int p, q;
    cin>>p>>q;
    couples[p]=q; 
    couples[q]=p;
  }
  cin>>n;
  for(int i=0; i<n; i++){
    int t;
    cin>>t;
    visited[t]=1;
    single.push_back(t);
  }
  for(int i=0; i<n; i++)
    if(visited[couples[single[i]]]==0)
      ans.push_back(single[i]);
  sort(ans.begin(), ans.end());
  cout<<ans.size()<<endl;
  for(int i=0; i<ans.size(); i++)
    i==0?printf("%05d", ans[i]):printf(" %05d", ans[i]);
  return 0;
}