Problem p04030 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
#define trace1(a) cout << #a << ": " << a << endl
#define trace2(a, b) cout << #a << ": " << a << " | ", trace1(b)
#define trace3(a, b, c) cout << #a << ": " << a << " | ", trace2(b, c)
#define trace4(a, b, c, d) cout << #a << ": " << a << " | ", trace3(b, c, d)
#define trace5(a, b, c, d, e)                                                  \
  cout << #a << ": " << a << " | ", trace4(b, c, d, e)
#define trace6(a, b, c, d, e, f)                                               \
  cout << #a << ": " << a << " | ", trace5(b, c, d, e, f)
#define trace7(a, b, c, d, e, f, g)                                            \
  cout << #a << ": " << a << " | ", trace6(b, c, d, e, f, g)
#define trace8(a, b, c, d, e, f, g, h)                                         \
  cout << #a << ": " << a << " | ", trace7(b, c, d, e, f, g, h)
#define trace9(a, b, c, d, e, f, g, h, i)                                      \
  cout << #a << ": " << a << " | ", trace8(b, c, d, e, f, g, h, i)
#define trace10(a, b, c, d, e, f, g, h, i, j)                                  \
  cout << #a << ": " << a << " | ", trace9(b, c, d, e, f, g, h, i, j)
#define rep(i, a, b) for (int(i) = (int)(a); (i) < (int)(b); ++(i))
#define reps(i, a, b, c) for (int(i) = (int)(a); (i) < (int)(b); (i) += (c))
#define rrep(i, a, b) for (int(i) = (int)(a); (i) >= (int)(b); --(i))
#define rreps(i, a, b, c) for (int(i) = (int)(a); (i) >= (int)(b); (i) -= (c))
#define fore(x, a) for (auto &x : a)
#define foreach(i, a) for (auto i = (a).begin(); i != (a).end(); ++i)
#define rforeach(i, a) for (auto i = (a).rbegin(); i != (a).rend(); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define isin(i, a, b) ((a) <= (i) && (i) <= (b))
#define uni(a) (a).erase(unique(all(a)), (a).end())
#define dup(x, y) (((x) + (y)-1) / (y))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(a) ((long long)(a).size())
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
using ll = long long;
using vi = vector<int>;
using unit = unsigned;
using vl = vector<ll>;
using ull = unsigned long long;
using vvi = vector<vi>;
using vvl = vector<vl>;
using pii = pair<int, int>;
using vpii = vector<pii>;
using pil = pair<int, ll>;
using vpil = vector<pil>;
using pli = pair<ll, int>;
using vpli = vector<pli>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  _main();
}
template <class T> string join(const v(T) & v) {
  stringstream s;
  rep(i, 0, sz(v)) s << ' ' << v[i];
  return s.str().substr(1);
}
template <class T> istream &operator>>(istream &i, v(T) & v) {
  fore(x, v) { i >> v; }
  return i;
}
template <class T> ostream &operator<<(ostream &o, const v(T) & v) {
  o << "[";
  fore(x, v) o << x << ",";
  o << "]";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const deque<T> &v) {
  o << "deq[";
  fore(x, v) o << x << ",";
  o << "]";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const set<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const unordered_set<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const multiset<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T>
ostream &operator<<(ostream &o, const unordered_multiset<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T1, class T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  o << "(" << p.fi << "," << p.se << ")";
  return o;
}
template <class TK, class TV>
ostream &operator<<(ostream &o, const map<TK, TV> &m) {
  o << "{";
  fore(x, m) o << x.fi << "=>" << x.se << ",";
  o << "}";
  return o;
}
template <class TK, class TV>
ostream &operator<<(ostream &o, const unordered_map<TK, TV> &m) {
  o << "{";
  fore(x, m) o << x.fi << "=>" << x.se << ",";
  o << "}";
  return o;
}
template <class T> void YES(T c) {
  if (c)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> void Yes(T c) {
  if (c)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> void POSS(T c) {
  if (c)
    cout << "POSSIBLE" << endl;
  else
    cout << "IMPOSSIBLE" << endl;
}
template <class T> void Poss(T c) {
  if (c)
    cout << "Possible" << endl;
  else
    cout << "Impossible" << endl;
}
template <class T> void chmax(T &a, const T &b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, const T &b) {
  if (b < a)
    a = b;
}
template <class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = 1001002003;
const ll LINF = 1001002003004005006LL;
const int dx[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};

void _main() {
  string s;
  cin >> s;
  string ans;
  fore(x, s) {
    if (x == 'B') {
      ans.pop_back();
    } else {
      ans.pb(x);
    }
  }
  cout << ans << endl;
}


Fixed submission:
#include <bits/stdc++.h>
#define trace1(a) cout << #a << ": " << a << endl
#define trace2(a, b) cout << #a << ": " << a << " | ", trace1(b)
#define trace3(a, b, c) cout << #a << ": " << a << " | ", trace2(b, c)
#define trace4(a, b, c, d) cout << #a << ": " << a << " | ", trace3(b, c, d)
#define trace5(a, b, c, d, e)                                                  \
  cout << #a << ": " << a << " | ", trace4(b, c, d, e)
#define trace6(a, b, c, d, e, f)                                               \
  cout << #a << ": " << a << " | ", trace5(b, c, d, e, f)
#define trace7(a, b, c, d, e, f, g)                                            \
  cout << #a << ": " << a << " | ", trace6(b, c, d, e, f, g)
#define trace8(a, b, c, d, e, f, g, h)                                         \
  cout << #a << ": " << a << " | ", trace7(b, c, d, e, f, g, h)
#define trace9(a, b, c, d, e, f, g, h, i)                                      \
  cout << #a << ": " << a << " | ", trace8(b, c, d, e, f, g, h, i)
#define trace10(a, b, c, d, e, f, g, h, i, j)                                  \
  cout << #a << ": " << a << " | ", trace9(b, c, d, e, f, g, h, i, j)
#define rep(i, a, b) for (int(i) = (int)(a); (i) < (int)(b); ++(i))
#define reps(i, a, b, c) for (int(i) = (int)(a); (i) < (int)(b); (i) += (c))
#define rrep(i, a, b) for (int(i) = (int)(a); (i) >= (int)(b); --(i))
#define rreps(i, a, b, c) for (int(i) = (int)(a); (i) >= (int)(b); (i) -= (c))
#define fore(x, a) for (auto &x : a)
#define foreach(i, a) for (auto i = (a).begin(); i != (a).end(); ++i)
#define rforeach(i, a) for (auto i = (a).rbegin(); i != (a).rend(); ++i)
#define all(a) (a).begin(), (a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define isin(i, a, b) ((a) <= (i) && (i) <= (b))
#define uni(a) (a).erase(unique(all(a)), (a).end())
#define dup(x, y) (((x) + (y)-1) / (y))
#define fi first
#define se second
#define pb push_back
#define eb emplace_back
#define sz(a) ((long long)(a).size())
#define v(T) vector<T>
#define vv(T) v(v(T))
using namespace std;
using ll = long long;
using vi = vector<int>;
using unit = unsigned;
using vl = vector<ll>;
using ull = unsigned long long;
using vvi = vector<vi>;
using vvl = vector<vl>;
using pii = pair<int, int>;
using vpii = vector<pii>;
using pil = pair<int, ll>;
using vpil = vector<pil>;
using pli = pair<ll, int>;
using vpli = vector<pli>;
using pll = pair<ll, ll>;
using vpll = vector<pll>;
void _main();
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  _main();
}
template <class T> string join(const v(T) & v) {
  stringstream s;
  rep(i, 0, sz(v)) s << ' ' << v[i];
  return s.str().substr(1);
}
template <class T> istream &operator>>(istream &i, v(T) & v) {
  fore(x, v) { i >> v; }
  return i;
}
template <class T> ostream &operator<<(ostream &o, const v(T) & v) {
  o << "[";
  fore(x, v) o << x << ",";
  o << "]";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const deque<T> &v) {
  o << "deq[";
  fore(x, v) o << x << ",";
  o << "]";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const set<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const unordered_set<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T> ostream &operator<<(ostream &o, const multiset<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T>
ostream &operator<<(ostream &o, const unordered_multiset<T> &v) {
  o << "{";
  fore(x, v) o << x << ",";
  o << "}";
  return o;
}
template <class T1, class T2>
ostream &operator<<(ostream &o, const pair<T1, T2> &p) {
  o << "(" << p.fi << "," << p.se << ")";
  return o;
}
template <class TK, class TV>
ostream &operator<<(ostream &o, const map<TK, TV> &m) {
  o << "{";
  fore(x, m) o << x.fi << "=>" << x.se << ",";
  o << "}";
  return o;
}
template <class TK, class TV>
ostream &operator<<(ostream &o, const unordered_map<TK, TV> &m) {
  o << "{";
  fore(x, m) o << x.fi << "=>" << x.se << ",";
  o << "}";
  return o;
}
template <class T> void YES(T c) {
  if (c)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
}
template <class T> void Yes(T c) {
  if (c)
    cout << "Yes" << endl;
  else
    cout << "No" << endl;
}
template <class T> void POSS(T c) {
  if (c)
    cout << "POSSIBLE" << endl;
  else
    cout << "IMPOSSIBLE" << endl;
}
template <class T> void Poss(T c) {
  if (c)
    cout << "Possible" << endl;
  else
    cout << "Impossible" << endl;
}
template <class T> void chmax(T &a, const T &b) {
  if (a < b)
    a = b;
}
template <class T> void chmin(T &a, const T &b) {
  if (b < a)
    a = b;
}
template <class T> T gcd(T a, T b) { return b ? gcd(b, a % b) : a; }
template <class T> T lcm(T a, T b) { return a / gcd(a, b) * b; }
const double EPS = 1e-10;
const double PI = acos(-1.0);
const int INF = 1001002003;
const ll LINF = 1001002003004005006LL;
const int dx[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int dy[] = {0, 1, 0, -1, -1, 1, -1, 1};

void _main() {
  string s;
  cin >> s;
  string ans;
  fore(x, s) {
    if (x == 'B') {
      if (!ans.empty())
        ans.pop_back();
    } else {
      ans.pb(x);
    }
  }
  cout << ans << endl;
}