Problem p03761 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>

#define ll long long
#define ull unsigned long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(a, b) for (int a = 0; a < (b); ++a)
#define REP1(i, n) for (int i = 1; i <= (n); ++i)
#define debug(x) cerr << #x << ": " << x << '\n'
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define pb push_back
#define mp make_pair
#define INF (1 << 29)
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;

int dy[] = {0, 0, 1, -1, 0};
int dx[] = {1, -1, 0, 0, 0};

template <class T = int> T in() {
  T x;
  cin >> x;
  return (x);
}
template <class T> void print(T &x) { cout << x << '\n'; }

const int MOD = (int)1e9 + 7;
const int MAX = 510000;
ll fac[MAX], finv[MAX], inv[MAX];
void COMint() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  if (a > b) {
    swap(a, b);
  }
  return gcd(a, b % a);
}
ll lcm(ll a, ll b) {
  ll g;
  g = gcd(a, b);
  return a * b / g;
}
bool compare_by_b(pair<int, int> a, pair<int, int> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}

bool compare_by_a(pair<int, int> a, pair<int, int> b) {
  if (a.first != b.first) {
    return a.first < b.first;
  } else {
    return a.second < b.second;
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n = in();
  string s;
  int ans[24];
  REP(i, 24) { ans[i] = 100; }
  REP(i, n) {
    cin >> s;
    vector<int> str;
    REP(j, s.size()) { str.pb(s[j] - 'a'); }
    REP(j, 26) { ans[j] = min(ans[j], int(count(all(str), j))); }
  }
  REP(i, 26) {
    if (ans[i] == 100)
      continue;
    REP(j, ans[i]) { cout << char('a' + i); }
  }
  cout << endl;
  return 0;
}

Fixed submission:
#include <bits/stdc++.h>

#define ll long long
#define ull unsigned long long
#define FOR(i, a, b) for (int i = (a); i < (b); ++i)
#define REP(a, b) for (int a = 0; a < (b); ++a)
#define REP1(i, n) for (int i = 1; i <= (n); ++i)
#define debug(x) cerr << #x << ": " << x << '\n'
#define all(x) (x).begin(), (x).end()
#define SZ(x) ((int)(x).size())
#define pb push_back
#define mp make_pair
#define INF (1 << 29)
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;

int dy[] = {0, 0, 1, -1, 0};
int dx[] = {1, -1, 0, 0, 0};

template <class T = int> T in() {
  T x;
  cin >> x;
  return (x);
}
template <class T> void print(T &x) { cout << x << '\n'; }

const int MOD = (int)1e9 + 7;
const int MAX = 510000;
ll fac[MAX], finv[MAX], inv[MAX];
void COMint() {
  fac[0] = fac[1] = 1;
  finv[0] = finv[1] = 1;
  inv[1] = 1;
  for (int i = 2; i < MAX; i++) {
    fac[i] = fac[i - 1] * i % MOD;
    inv[i] = MOD - inv[MOD % i] * (MOD / i) % MOD;
    finv[i] = finv[i - 1] * inv[i] % MOD;
  }
}
ll COM(int n, int k) {
  if (n < k)
    return 0;
  if (n < 0 || k < 0)
    return 0;
  return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  if (a > b) {
    swap(a, b);
  }
  return gcd(a, b % a);
}
ll lcm(ll a, ll b) {
  ll g;
  g = gcd(a, b);
  return a * b / g;
}
bool compare_by_b(pair<int, int> a, pair<int, int> b) {
  if (a.second != b.second) {
    return a.second < b.second;
  } else {
    return a.first < b.first;
  }
}

bool compare_by_a(pair<int, int> a, pair<int, int> b) {
  if (a.first != b.first) {
    return a.first < b.first;
  } else {
    return a.second < b.second;
  }
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n = in();
  string s;
  int ans[26];
  REP(i, 26) { ans[i] = 100; }
  REP(i, n) {
    cin >> s;
    vector<int> str;
    REP(j, s.size()) { str.pb(s[j] - 'a'); }
    REP(j, 26) { ans[j] = min(ans[j], int(count(all(str), j))); }
  }
  REP(i, 26) {
    if (ans[i] == 100)
      continue;
    REP(j, ans[i]) { cout << char('a' + i); }
  }
  cout << endl;
  return 0;
}