Problem p04030 (C++)

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

typedef long long int ll;
#define all(x) (x).begin(), (x).end()

using namespace std;

int nxt() {
  int x;
  cin >> x;
  return x;
}

ll nxtl() {
  ll x;
  cin >> x;
  return x;
}

void SieveOfEratosthenes(int n, vector<int> &primes) {
  // Create a boolean array "prime[0..n]" and initialize
  // all entries it as true. A value in prime[i] will
  // finally be false if i is Not a prime, else true.
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));

  for (int p = 2; p * p <= n; p++) {
    // If prime[p] is not changed, then it is a prime
    if (prime[p] == true) {
      // Update all multiples of p greater than or
      // equal to the square of it
      // numbers which are multiple of p and are
      // less than p^2 are already been marked.
      for (int i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }

  // Print all prime numbers
  for (int p = 2; p <= n; p++)
    if (prime[p])
      primes.push_back(p);
}

ll max(ll a, ll b) {
  if (a > b)
    return a;
  return b;
}

ll power(ll x, ll y, ll mod) {
  ll temp;
  if (y == 0)
    return 1;
  temp = power(x, y / 2, mod);
  if (y % 2 == 0)
    return (temp * temp) % mod;
  else
    return (((x * temp) % mod) * temp) % mod;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  int n = s.length();
  stack<char> st;
  for (int i = 0; i < n; i++) {
    if (s[i] != 'B')
      st.push(s[i]);
    else
      st.pop();
  }
  vector<char> fin;
  while (st.size()) {
    fin.push_back(st.top());
    st.pop();
  }
  reverse(all(fin));
  for (auto i : fin)
    cout << i;
  cout << endl;

  return 0;
}


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

typedef long long int ll;
#define all(x) (x).begin(), (x).end()

using namespace std;

int nxt() {
  int x;
  cin >> x;
  return x;
}

ll nxtl() {
  ll x;
  cin >> x;
  return x;
}

void SieveOfEratosthenes(int n, vector<int> &primes) {
  // Create a boolean array "prime[0..n]" and initialize
  // all entries it as true. A value in prime[i] will
  // finally be false if i is Not a prime, else true.
  bool prime[n + 1];
  memset(prime, true, sizeof(prime));

  for (int p = 2; p * p <= n; p++) {
    // If prime[p] is not changed, then it is a prime
    if (prime[p] == true) {
      // Update all multiples of p greater than or
      // equal to the square of it
      // numbers which are multiple of p and are
      // less than p^2 are already been marked.
      for (int i = p * p; i <= n; i += p)
        prime[i] = false;
    }
  }

  // Print all prime numbers
  for (int p = 2; p <= n; p++)
    if (prime[p])
      primes.push_back(p);
}

ll max(ll a, ll b) {
  if (a > b)
    return a;
  return b;
}

ll power(ll x, ll y, ll mod) {
  ll temp;
  if (y == 0)
    return 1;
  temp = power(x, y / 2, mod);
  if (y % 2 == 0)
    return (temp * temp) % mod;
  else
    return (((x * temp) % mod) * temp) % mod;
}

int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  string s;
  cin >> s;
  int n = s.length();
  stack<char> st;
  for (int i = 0; i < n; i++) {
    if (s[i] != 'B')
      st.push(s[i]);
    else {
      if (st.size())
        st.pop();
    }
  }
  vector<char> fin;
  while (st.size()) {
    fin.push_back(st.top());
    st.pop();
  }
  reverse(all(fin));
  for (auto i : fin)
    cout << i;
  cout << endl;

  return 0;
}