Problem p03408 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < n; i++)
using namespace std;
using ll = long long;
using P = pair<int, int>;

const int mod = 1000000007;
// const int mod = 998244353;

struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
  int n;
  cin >> n;
  vector<string> s(n);
  rep(i, 0, n) cin >> s[i];
  int m;
  cin >> m;
  vector<string> t(n);
  rep(i, 0, m) cin >> t[i];
  map<string, int> mp;
  rep(i, 0, n) mp[s[i]]++;
  rep(i, 0, m) mp[t[i]]--;
  int ans = 0;
  for (auto i : mp)
    ans = max(ans, i.second);
  cout << ans << endl;
  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define rep(i, a, n) for (int i = a; i < n; i++)
using namespace std;
using ll = long long;
using P = pair<int, int>;

const int mod = 1000000007;
// const int mod = 998244353;

struct mint {
  ll x;
  mint(ll x = 0) : x((x % mod + mod) % mod) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint a) {
    if ((x += a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator-=(const mint a) {
    if ((x += mod - a.x) >= mod)
      x -= mod;
    return *this;
  }
  mint &operator*=(const mint a) {
    (x *= a.x) %= mod;
    return *this;
  }
  mint operator+(const mint a) const { return mint(*this) += a; }
  mint operator-(const mint a) const { return mint(*this) -= a; }
  mint operator*(const mint a) const { return mint(*this) *= a; }
  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  // for prime mod
  mint inv() const { return pow(mod - 2); }
  mint &operator/=(const mint a) { return *this *= a.inv(); }
  mint operator/(const mint a) const { return mint(*this) /= a; }
};
istream &operator>>(istream &is, mint &a) { return is >> a.x; }
ostream &operator<<(ostream &os, const mint &a) { return os << a.x; }

ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

int main() {
  int n;
  cin >> n;
  vector<string> s(n);
  rep(i, 0, n) cin >> s[i];
  int m;
  cin >> m;
  vector<string> t(m);
  rep(i, 0, m) cin >> t[i];
  map<string, int> mp;
  rep(i, 0, n) mp[s[i]]++;
  rep(i, 0, m) mp[t[i]]--;
  int ans = 0;
  for (auto i : mp)
    ans = max(ans, i.second);
  cout << ans << endl;
  return 0;
}