HashTable Again!

#include <iostream>
#include <vector>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>

using namespace std;
int n, m, u, v;

int main(int argc, char** argv) {
  cin >> n;
  unordered_map<int, int> mp;
  while (n--) {
    cin >> u >> v;
    mp[u] = v;
    mp[v] = u;
  }

  cin >> m;
  vector<int> ids(m);
  for (int i = 0; i < m; ++i) cin >> ids[i];
  unordered_set<int> s(begin(ids), end(ids));

  vector<int> ans;
  for (const int id : ids)
    if (!mp.count(id) || !s.count(mp.at(id)))
      ans.emplace_back(id); // 没有女朋友或没来参加大型派对

  sort(begin(ans), end(ans));
  printf("%d\n", ans.size());
  for (int i = 0; i < ans.size(); ++i) {
    printf("%05d", ans[i]);
    if (i < ans.size() - 1) putchar(' ');
  }

  return 0;
}