Problem p04030 (C++)

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

using namespace std;

#define int long long
#define ar array
#define vt vector
#define pb push_back
#define all(c) (c).begin(), (c).end()
#define sz(x) (int)(x).size()

#define F_OR(i, a, b, s)                                                       \
  for (int i = (a); (s) > 0 ? i < (b) : i > (b); i += (s))
#define F_OR1(e) F_OR(i, 0, e, 1)
#define F_OR2(i, e) F_OR(i, 0, e, 1)
#define F_OR3(i, b, e) F_OR(i, b, e, 1)
#define F_OR4(i, b, e, s) F_OR(i, b, e, s)
#define GET5(a, b, c, d, e, ...) e
#define F_ORC(...) GET5(__VA_ARGS__, F_OR4, F_OR3, F_OR2, F_OR1)
#define FOR(...) F_ORC(__VA_ARGS__)(__VA_ARGS__)
#define EACH(x, a) for (auto &x : a)

template <class T> bool umin(T &a, const T &b) { return b < a ? a = b, 1 : 0; }

template <class T> bool umax(T &a, const T &b) { return a < b ? a = b, 1 : 0; }

template <class A> void read(vt<A> &v);

template <class A, size_t S> void read(ar<A, S> &a);

template <class T> void read(T &x) { cin >> x; }

void read(double &d) {
  string t;
  read(t);
  d = stod(t);
}

void read(long double &d) {
  string t;
  read(t);
  d = stold(t);
}

template <class H, class... T> void read(H &h, T &...t) {
  read(h);
  read(t...);
}

template <class A> void read(vt<A> &x) { EACH(a, x) read(a); }

template <class A, size_t S> void read(array<A, S> &x) { EACH(a, x) read(a); }

string to_string(char c) { return string(1, c); }

string to_string(bool b) { return b ? "true" : "false"; }

string to_string(const char *s) { return string(s); }

string to_string(string s) { return s; }

string to_string(vt<bool> v) {
  string res;
  FOR(sz(v)) res += char('0' + v[i]);
  return res;
}

template <size_t S> string to_string(bitset<S> b) {
  string res;
  FOR(S) res += char('0' + b[i]);
  return res;
}

template <class T> string to_string(T v) {
  bool f = 1;
  string res;
  EACH(x, v) {
    if (!f)
      res += ' ';
    f = 0;
    res += to_string(x);
  }
  return res;
}

template <class A> void write(A x) { cout << to_string(x); }

template <class H, class... T> void write(const H &h, const T &...t) {
  write(h);
  write(t...);
}

void print() { write("\n"); }

template <class H, class... T> void print(const H &h, const T &...t) {
  write(h);
  if (sizeof...(t))
    write(' ');
  print(t...);
}

template <class T, class U> void vti(vt<T> &v, U x, size_t n) {
  v = vt<T>(n, x);
}

template <class T, class U> void vti(vt<T> &v, U x, size_t n, size_t m...) {
  v = vt<T>(n);
  EACH(a, v) vti(a, x, m);
}

int cmp(int a, int b) {
  if (a == b) {
    return 0;
  }
  return a < b ? -1 : 1;
}

void solve() {
  string s;
  read(s);
  string out;
  EACH(x, s) {
    if (x == '1' || x == '0') {
      out += x;
    } else {
      out.pop_back();
    }
  }
  print(out);
}

int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  int tt = 1;
  //    read(tt);
  FOR(tt) {
    //        write("Case #", i + 1, ": ");
    solve();
  }
}


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

using namespace std;

#define int long long
#define ar array
#define vt vector
#define pb push_back
#define all(c) (c).begin(), (c).end()
#define sz(x) (int)(x).size()

#define F_OR(i, a, b, s)                                                       \
  for (int i = (a); (s) > 0 ? i < (b) : i > (b); i += (s))
#define F_OR1(e) F_OR(i, 0, e, 1)
#define F_OR2(i, e) F_OR(i, 0, e, 1)
#define F_OR3(i, b, e) F_OR(i, b, e, 1)
#define F_OR4(i, b, e, s) F_OR(i, b, e, s)
#define GET5(a, b, c, d, e, ...) e
#define F_ORC(...) GET5(__VA_ARGS__, F_OR4, F_OR3, F_OR2, F_OR1)
#define FOR(...) F_ORC(__VA_ARGS__)(__VA_ARGS__)
#define EACH(x, a) for (auto &x : a)

template <class T> bool umin(T &a, const T &b) { return b < a ? a = b, 1 : 0; }

template <class T> bool umax(T &a, const T &b) { return a < b ? a = b, 1 : 0; }

template <class A> void read(vt<A> &v);

template <class A, size_t S> void read(ar<A, S> &a);

template <class T> void read(T &x) { cin >> x; }

void read(double &d) {
  string t;
  read(t);
  d = stod(t);
}

void read(long double &d) {
  string t;
  read(t);
  d = stold(t);
}

template <class H, class... T> void read(H &h, T &...t) {
  read(h);
  read(t...);
}

template <class A> void read(vt<A> &x) { EACH(a, x) read(a); }

template <class A, size_t S> void read(array<A, S> &x) { EACH(a, x) read(a); }

string to_string(char c) { return string(1, c); }

string to_string(bool b) { return b ? "true" : "false"; }

string to_string(const char *s) { return string(s); }

string to_string(string s) { return s; }

string to_string(vt<bool> v) {
  string res;
  FOR(sz(v)) res += char('0' + v[i]);
  return res;
}

template <size_t S> string to_string(bitset<S> b) {
  string res;
  FOR(S) res += char('0' + b[i]);
  return res;
}

template <class T> string to_string(T v) {
  bool f = 1;
  string res;
  EACH(x, v) {
    if (!f)
      res += ' ';
    f = 0;
    res += to_string(x);
  }
  return res;
}

template <class A> void write(A x) { cout << to_string(x); }

template <class H, class... T> void write(const H &h, const T &...t) {
  write(h);
  write(t...);
}

void print() { write("\n"); }

template <class H, class... T> void print(const H &h, const T &...t) {
  write(h);
  if (sizeof...(t))
    write(' ');
  print(t...);
}

template <class T, class U> void vti(vt<T> &v, U x, size_t n) {
  v = vt<T>(n, x);
}

template <class T, class U> void vti(vt<T> &v, U x, size_t n, size_t m...) {
  v = vt<T>(n);
  EACH(a, v) vti(a, x, m);
}

int cmp(int a, int b) {
  if (a == b) {
    return 0;
  }
  return a < b ? -1 : 1;
}

void solve() {
  string s;
  read(s);
  string out;
  EACH(x, s) {
    if (x == '1' || x == '0') {
      out += x;
    } else {
      if (out.size())
        out.pop_back();
    }
  }
  print(out);
}

int32_t main() {
  ios::sync_with_stdio(0);
  cin.tie(0);

  int tt = 1;
  //    read(tt);
  FOR(tt) {
    //        write("Case #", i + 1, ": ");
    solve();
  }
}