Problem p04030 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define rep(i, n, m) for (int i = (n); i < (m); i++)
#define rep_d(i, n, m) for (int i = (n)-1; i >= (m); i--)
#define sort_asc(X) sort((X).begin(), (X).end())
#define sort_desc(X) sort((X).begin(), (X).end(), greater<>())

template <class T> bool chmax(T &a, T b);
template <class T> bool chmin(T &a, T b);
int *eratosthenes(int N);
bool bit_search(int pattern, int N);
template <class T> void cumulative_sum(T array, size_t N);

const ll INF = 1LL << 60;
const int k_mod = 1e9 + 7;

typedef struct {
  vector<int> to;
  int cost;
} edge;
typedef struct {
  bool a;
  bool b;
} P;
int main(void) {
  string s, str;
  cin >> s;
  int N = s.size();
  // char[N] = '\n';
  int index = 0;
  rep(i, 0, N) { str[i] = 0; }
  rep(i, 0, N) {
    if (s[i] == 'B') {
      str.pop_back();
    } else {
      str.push_back(s[i]);
    }
  }
  cout << str << endl;
}

int *eratosthenes(int N) {
  int *prime_array = new int[N + 1];
  int lim = N;
  prime_array[0] = 0;
  prime_array[1] = 0;
  rep(i, 2, N + 1) { prime_array[i] = 1; }
  rep(i, 2, lim) {
    if (prime_array[i] == 0)
      continue;
    lim = N / i;
    for (int j = i * 2; j < N + 1; j += i) {
      prime_array[j] = 0;
    }
  }
  return prime_array;
}
template <class T> void cumulative_sum(T array, size_t N) {
  rep(i, 1, N + 1) { array[i] += array[i - 1]; }
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
bool bit_search(int pattern, int N) {
  int cnt = 0;
  rep(bit, 0, N) {
    if (pattern & (1 << bit)) {
      cnt++;
    }
  }
  return true;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
#define rep(i, n, m) for (int i = (n); i < (m); i++)
#define rep_d(i, n, m) for (int i = (n)-1; i >= (m); i--)
#define sort_asc(X) sort((X).begin(), (X).end())
#define sort_desc(X) sort((X).begin(), (X).end(), greater<>())

template <class T> bool chmax(T &a, T b);
template <class T> bool chmin(T &a, T b);
int *eratosthenes(int N);
bool bit_search(int pattern, int N);
template <class T> void cumulative_sum(T array, size_t N);

const ll INF = 1LL << 60;
const int k_mod = 1e9 + 7;

typedef struct {
  vector<int> to;
  int cost;
} edge;
typedef struct {
  bool a;
  bool b;
} P;
int main(void) {
  string s, str;
  cin >> s;
  int N = s.size();
  // char[N] = '\n';
  int index = 0;
  rep(i, 0, N) { str[i] = 0; }
  rep(i, 0, N) {
    if (s[i] == 'B') {
      if (str.size() != 0)
        str.pop_back();
    } else {
      str.push_back(s[i]);
    }
  }
  cout << str << endl;
}

int *eratosthenes(int N) {
  int *prime_array = new int[N + 1];
  int lim = N;
  prime_array[0] = 0;
  prime_array[1] = 0;
  rep(i, 2, N + 1) { prime_array[i] = 1; }
  rep(i, 2, lim) {
    if (prime_array[i] == 0)
      continue;
    lim = N / i;
    for (int j = i * 2; j < N + 1; j += i) {
      prime_array[j] = 0;
    }
  }
  return prime_array;
}
template <class T> void cumulative_sum(T array, size_t N) {
  rep(i, 1, N + 1) { array[i] += array[i - 1]; }
}
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return true;
  }
  return false;
}
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return true;
  }
  return false;
}
bool bit_search(int pattern, int N) {
  int cnt = 0;
  rep(bit, 0, N) {
    if (pattern & (1 << bit)) {
      cnt++;
    }
  }
  return true;
}