Problem p04030 (C++)

Buggy submission (status: Runtime Error):
#include <algorithm>
#include <array>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <limits.h>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <vector>
#define rep(i, s, n) for (int i = (s); (n) > i; i++)
#define REP(i, n) rep(i, 0, n)
#define RANGE(x, a, b) ((a) <= (x) && (x) <= (b))
#define DUPLE(a, b, c, d)                                                      \
  (RANGE(a, c, d) || RANGE(b, c, d) || RANGE(c, a, b) || RANGE(d, a, b))
#define INCLU(a, b, c, d) (RANGE(a, c, d) && (b, c, d))
#define PW(x) ((x) * (x))
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define MODU 1000000007
#define bitcheck(a, b) ((a >> b) & 1)
#define bitset(a, b) (a |= (1 << b))
#define bitunset(a, b) (a &= ~(1 << b))
#define MP(a, b) make_pair((a), (b))
#define Manh(a, b) (abs((a).first-(b).first) + abs((a).second - ((b).second))
#define pritnf printf
#define scnaf scanf
#define itn int
#define PI 3.141592653589
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
pii Dir[] = {{1, 0}, {0, 1}};

signed main() {
  char str[1000000];
  scnaf("%s", str);
  string s;
  REP(i, strlen(str)) {
    if (str[i] == 'B')
      s.erase(--s.end());
    else
      s += str[i];
  }
  printf("%s\n", s.c_str());
  return 0;
}

Fixed submission:
#include <algorithm>
#include <array>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstring>
#include <functional>
#include <iostream>
#include <limits.h>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <unordered_map>
#include <vector>
#define rep(i, s, n) for (int i = (s); (n) > i; i++)
#define REP(i, n) rep(i, 0, n)
#define RANGE(x, a, b) ((a) <= (x) && (x) <= (b))
#define DUPLE(a, b, c, d)                                                      \
  (RANGE(a, c, d) || RANGE(b, c, d) || RANGE(c, a, b) || RANGE(d, a, b))
#define INCLU(a, b, c, d) (RANGE(a, c, d) && (b, c, d))
#define PW(x) ((x) * (x))
#define ALL(x) (x).begin(), (x).end()
#define RALL(x) (x).rbegin(), (x).rend()
#define MODU 1000000007
#define bitcheck(a, b) ((a >> b) & 1)
#define bitset(a, b) (a |= (1 << b))
#define bitunset(a, b) (a &= ~(1 << b))
#define MP(a, b) make_pair((a), (b))
#define Manh(a, b) (abs((a).first-(b).first) + abs((a).second - ((b).second))
#define pritnf printf
#define scnaf scanf
#define itn int
#define PI 3.141592653589
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
ll gcd(ll a, ll b) {
  if (b == 0)
    return a;
  return gcd(b, a % b);
}
template <typename A, size_t N, typename T>
void Fill(A (&array)[N], const T &val) {
  std::fill((T *)array, (T *)(array + N), val);
}
pii Dir[] = {{1, 0}, {0, 1}};

signed main() {
  char str[1000000];
  scnaf("%s", str);
  string s;
  REP(i, strlen(str)) {
    if (str[i] == 'B') {
      if (!s.empty())
        s.erase(--s.end());
    } else
      s += str[i];
  }
  printf("%s\n", s.c_str());
  return 0;
}