https://www.acmicpc.net/problem/1764
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define X first
#define Y second
#define pb push_back
#define fastio cin.tie(0)->sync_with_stdio(0)
#define sz(v) (int)(v).size()
#define all(v) v.begin(), v.end()
#define rall(v) (v).rbegin(), (v).rend()
#define compress(v) sort(all(v)), (v).erase(unique(all(v)), (v).end())
#define OOB(x, y) ((x) < 0 || (x) >= n || (y) < 0 || (y) >= m)
#define debug(x) cout << (#x) << ": " << (x) << '\n'
using namespace std;
using ll = long long;
using ull = unsigned long long;
using dbl = double;
using ldb = long double;
using pii = pair<int,int>;
using pll = pair<ll,ll>;
using vi = vector<int>;
using vs = vector<string>;
using tii = tuple<int,int,int>;
template<typename T> using wector = vector<vector<T>>;
const int MOD = 1e9 + 7;
const int INF = 1e9 + 7;
const ll LNF = 1e18 + 7;
int main() {
fastio;
set<string> M1; // M1 : 듣지 못함
vector<string> v;
int n,m; cin >> n >> m;
for(int i = 0; i < n; i++){
string s; cin >> s;
M1.insert(s);
}
for(int i = 0; i < m; i++){
string s; cin >> s;
if(M1.count(s)) v.pb(s);
}
sort(all(v));
cout << v.size() << "\n";
for(auto c : v) cout << c << "\n";
return 0;
}
Set을 이용해서 듣지 못한 사람을 저장한 뒤에 보지 못한 사람을 입력 받을 때 그 이름이 Set안에 있다면 벡터에 넣어서 정렬후에 출력해주면 됩니다.