Problem p02756 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>

// definitions
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define clr(x) memset(x, 0, sizeof(x))
#define rep(i, l, r) for (long long i = (l); i < (long long)(r); i++)
#define repb(i, r, l) for (long long i = (r); i > (long long)(l); i--)
#define pb push_back
#define pf push_front
#define mod1 998244353
#define mod 1000000007
#define MAX_CHAR 256

using namespace std;
typedef vector<long long> vl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<pair<long long, long long>> vp;
typedef vector<tuple<long long, long long, long long>> vtup;
typedef deque<long long> dql;
typedef deque<char> dqc;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;

// prevent collisions in unordered_map
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};

typedef unordered_map<long long, long long, custom_hash> umap;

// standard functions
ll gcd(ll x, ll y) {
  if (x < y)
    return gcd(y, x);
  if (y == 0)
    return x;
  return gcd(y, x % y);
}

ll mystoi(string str) {
  stringstream ss(str);
  ll ans = 0;
  ss >> ans;

  return ans;
}

ll exp(ll x, ll ex) {
  ll ans = 1ll;
  while (ex > 0) {
    if (ex & 1ll)
      ans = (ans * x);
    ex >>= 1ll;
    x = (x * x);
  }
  return ans;
}

ll nCrModp(ll n, ll r, ll p) {
  ll C[r + 1];
  memset(C, 0, sizeof(C));

  C[0] = 1;
  for (ll i = 1; i <= n; i++) {
    for (ll j = min(i, r); j > 0; j--)
      C[j] = (C[j] + C[j - 1]) % p;
  }
  return C[r];
}

bool isPrime(ll n) {
  if (n <= 1)
    return false;
  if (n <= 3)
    return true;
  if (n % 2 == 0 || n % 3 == 0)
    return false;

  for (ll i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0)
      return false;

  return true;
}

vl countFreq(ll arr[], ll n) {
  unordered_map<ll, ll> mp;
  vector<ll> v;

  for (ll i = 0; i < n; i++)
    mp[arr[i]]++;

  for (auto x : mp)
    v.pb(x.second);

  return v;
}

ll binarySearch(ll arr[], ll l, ll r, ll x) {
  while (l <= r) {
    ll m = l + (r - l) / 2;
    if (arr[m] == x)
      return m;
    if (arr[m] < x)
      l = m + 1;
    else
      r = m - 1;
  }
  return -1;
}

bool sort_cond(const pair<ll, ll> &a, const pair<ll, ll> &b) {
  return a.second > b.second;
}

const ll MAXSIEVE = (1e3);
ll spf[MAXSIEVE];
void sieve() {
  spf[1] = 1;
  for (ll i = 2; i < MAXSIEVE; i++)
    spf[i] = i;

  for (ll i = 4; i < MAXSIEVE; i += 2)
    spf[i] = 2;

  for (ll i = 3; i * i < MAXSIEVE; i++) {
    if (spf[i] == i) {
      for (ll j = i * i; j < MAXSIEVE; j += i)
        if (spf[j] == j)
          spf[j] = i;
    }
  }
}

// code beigns in main
int main() {

#ifndef ONLINE_JUDGE
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
#endif

  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ll t = 1;
  // cin >> t;

  while (t--) {
    string str;
    cin >> str;

    dqc start, end;
    ll q;
    cin >> q;

    ll rev = 0;
    ll count = 0;
    rep(i, 0, q) {
      ll type;
      cin >> type;
      if (type == 1) {
        count++;
        rev++;
      } else {
        ll ty;
        char ch;
        cin >> ty >> ch;
        if (rev % 2) {
          if (ty == 1)
            end.pb(ch);
          else
            start.pf(ch);
        } else {
          if (ty == 1)
            start.pf(ch);
          else
            end.pb(ch);
        }
      }
    }

    string ans = "";
    rep(i, 0, start.size()) ans += string(1, start[i]);

    ans += str;

    rep(i, 0, end.size()) ans += string(1, end[i]);

    if (rev % 2 == 1)
      reverse(all(ans));

    cout << ans << endl;
  }
}


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

// definitions
#define F first
#define S second
#define all(x) x.begin(), x.end()
#define clr(x) memset(x, 0, sizeof(x))
#define rep(i, l, r) for (long long i = (l); i < (long long)(r); i++)
#define repb(i, r, l) for (long long i = (r); i > (long long)(l); i--)
#define pb push_back
#define pf push_front
#define mod1 998244353
#define mod 1000000007
#define MAX_CHAR 256

using namespace std;
typedef vector<long long> vl;
typedef vector<char> vc;
typedef vector<string> vs;
typedef vector<pair<long long, long long>> vp;
typedef vector<tuple<long long, long long, long long>> vtup;
typedef deque<long long> dql;
typedef deque<char> dqc;
typedef long double ld;
typedef long long ll;
typedef unsigned long long ull;

// prevent collisions in unordered_map
struct custom_hash {
  static uint64_t splitmix64(uint64_t x) {
    x += 0x9e3779b97f4a7c15;
    x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
    x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
    return x ^ (x >> 31);
  }

  size_t operator()(uint64_t x) const {
    static const uint64_t FIXED_RANDOM =
        chrono::steady_clock::now().time_since_epoch().count();
    return splitmix64(x + FIXED_RANDOM);
  }
};

typedef unordered_map<long long, long long, custom_hash> umap;

// standard functions
ll gcd(ll x, ll y) {
  if (x < y)
    return gcd(y, x);
  if (y == 0)
    return x;
  return gcd(y, x % y);
}

ll mystoi(string str) {
  stringstream ss(str);
  ll ans = 0;
  ss >> ans;

  return ans;
}

ll exp(ll x, ll ex) {
  ll ans = 1ll;
  while (ex > 0) {
    if (ex & 1ll)
      ans = (ans * x);
    ex >>= 1ll;
    x = (x * x);
  }
  return ans;
}

ll nCrModp(ll n, ll r, ll p) {
  ll C[r + 1];
  memset(C, 0, sizeof(C));

  C[0] = 1;
  for (ll i = 1; i <= n; i++) {
    for (ll j = min(i, r); j > 0; j--)
      C[j] = (C[j] + C[j - 1]) % p;
  }
  return C[r];
}

bool isPrime(ll n) {
  if (n <= 1)
    return false;
  if (n <= 3)
    return true;
  if (n % 2 == 0 || n % 3 == 0)
    return false;

  for (ll i = 5; i * i <= n; i = i + 6)
    if (n % i == 0 || n % (i + 2) == 0)
      return false;

  return true;
}

vl countFreq(ll arr[], ll n) {
  unordered_map<ll, ll> mp;
  vector<ll> v;

  for (ll i = 0; i < n; i++)
    mp[arr[i]]++;

  for (auto x : mp)
    v.pb(x.second);

  return v;
}

ll binarySearch(ll arr[], ll l, ll r, ll x) {
  while (l <= r) {
    ll m = l + (r - l) / 2;
    if (arr[m] == x)
      return m;
    if (arr[m] < x)
      l = m + 1;
    else
      r = m - 1;
  }
  return -1;
}

bool sort_cond(const pair<ll, ll> &a, const pair<ll, ll> &b) {
  return a.second > b.second;
}

const ll MAXSIEVE = (1e3);
ll spf[MAXSIEVE];
void sieve() {
  spf[1] = 1;
  for (ll i = 2; i < MAXSIEVE; i++)
    spf[i] = i;

  for (ll i = 4; i < MAXSIEVE; i += 2)
    spf[i] = 2;

  for (ll i = 3; i * i < MAXSIEVE; i++) {
    if (spf[i] == i) {
      for (ll j = i * i; j < MAXSIEVE; j += i)
        if (spf[j] == j)
          spf[j] = i;
    }
  }
}

// code beigns in main
int main() {

  // #ifndef ONLINE_JUDGE
  // freopen("input.txt", "r", stdin);
  // freopen("output.txt", "w", stdout);
  // #endif

  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ll t = 1;
  // cin >> t;

  while (t--) {
    string str;
    cin >> str;

    dqc start, end;
    ll q;
    cin >> q;

    ll rev = 0;
    ll count = 0;
    rep(i, 0, q) {
      ll type;
      cin >> type;
      if (type == 1) {
        count++;
        rev++;
      } else {
        ll ty;
        char ch;
        cin >> ty >> ch;
        if (rev % 2) {
          if (ty == 1)
            end.pb(ch);
          else
            start.pf(ch);
        } else {
          if (ty == 1)
            start.pf(ch);
          else
            end.pb(ch);
        }
      }
    }

    string ans = "";
    rep(i, 0, start.size()) ans += string(1, start[i]);

    ans += str;

    rep(i, 0, end.size()) ans += string(1, end[i]);

    if (rev % 2 == 1)
      reverse(all(ans));

    cout << ans << endl;
  }
}