Problem p04030 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl '\n'
#define ll long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF 2147483647
#define LLINF 9223372036854775807LL
#define vi vector<int>
#define vvi vector<vector<int>>
#define pi pair<int, int>
#define ALL(a) (a).begin(), (a).end()
#define BIT(n) (1LL << (n))
#define TOUPPER(s) transform(s.begin(), s.end(), s.begin(), ::toupper)
#define TOLOWER(s) transform(s.begin(), s.end(), s.begin(), ::tolower)
#define FILLZERO(s, n)                                                         \
  do {                                                                         \
    ostringstream os;                                                          \
    os << setw(n) << setfill('0') << s;                                        \
    s = os.str();                                                              \
  } while (0);
#define UNIQUE(v)                                                              \
  v.erase(unique(v.begin(), v.end()), v.end()) // sortしてからつかうこと
#define INIT                                                                   \
  std::cin.tie(nullptr);                                                       \
  std::ios_base::sync_with_stdio(false);                                       \
  cout << std::fixed << std::setprecision(10);

template <typename T> vector<T> make_vec(size_t a, T n) {
  return vector<T>(a, n);
}
template <typename T, typename... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

constexpr ll MOD = 1e9 + 7;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

signed main() {
  INIT;
  cin >> S;
  REP(i, S.size()) {
    if (S[i] == 'B') {
      T.pop_back();
    } else
      T.push_back(S[i]);
  }
  cout << T << endl;
}

Fixed submission:
#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define dump(...)
#endif
#define endl '\n'
#define ll long long
#define FOR(i, a, b) for (int i = (int)(a); i < (int)(b); i++)
#define REP(i, x) for (int i = 0; i < (int)(x); i++)
#define REPS(i, x) for (int i = 1; i <= (int)(x); i++)
#define RREP(i, x) for (int i = ((int)(x)-1); i >= 0; i--)
#define RREPS(i, x) for (int i = ((int)(x)); i > 0; i--)
#define INF 2147483647
#define LLINF 9223372036854775807LL
#define vi vector<int>
#define vvi vector<vector<int>>
#define pi pair<int, int>
#define ALL(a) (a).begin(), (a).end()
#define BIT(n) (1LL << (n))
#define TOUPPER(s) transform(s.begin(), s.end(), s.begin(), ::toupper)
#define TOLOWER(s) transform(s.begin(), s.end(), s.begin(), ::tolower)
#define FILLZERO(s, n)                                                         \
  do {                                                                         \
    ostringstream os;                                                          \
    os << setw(n) << setfill('0') << s;                                        \
    s = os.str();                                                              \
  } while (0);
#define UNIQUE(v)                                                              \
  v.erase(unique(v.begin(), v.end()), v.end()) // sortしてからつかうこと
#define INIT                                                                   \
  std::cin.tie(nullptr);                                                       \
  std::ios_base::sync_with_stdio(false);                                       \
  cout << std::fixed << std::setprecision(10);

template <typename T> vector<T> make_vec(size_t a, T n) {
  return vector<T>(a, n);
}
template <typename T, typename... Ts> auto make_vec(size_t a, Ts... ts) {
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}
template <class T> inline bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> inline bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T> istream &operator>>(istream &is, vector<T> &vec) {
  for (T &x : vec)
    is >> x;
  return is;
}

constexpr ll MOD = 1e9 + 7;
int dx[8] = {1, 0, -1, 0, 1, 1, -1, -1};
int dy[8] = {0, 1, 0, -1, 1, -1, 1, -1};
ll A, B, C, D, E, F, G, H, N, M, L, K, P, Q, R, W, X, Y, Z;
string S, T;
ll ans = 0;

signed main() {
  INIT;
  cin >> S;
  REP(i, S.size()) {
    if (S[i] == 'B') {
      if (T.size() != 0)
        T.pop_back();
    } else
      T.push_back(S[i]);
  }
  cout << T << endl;
}