Problem p04030 (C++)

Buggy submission (status: Runtime Error):
/*

 ALLAH IS ALMIGHTY              ******BISMILLAHIR RAHMANIR RAHIM.****** ALLAH IS
 ALMIGHTY
                                    ```````````````````````````````
                                        AUTHOR: ANISUR RAHAMAN
                                            BSMRSTU(SHIICT)
                                                ID:017

                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                           ****!!!!!!!!****
                                            **************
                                             ************
                                              **********
                                               ********
                                                ******
                                                 ****
                                                  **

 _____________________________________________________________________________________________________
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 `````````````````````````````````````````````````````````````````````````````````````````````````````
 ****** ******
 ***** *****
 **** ****
 *** ***
 ** **
 * *
         $$$$$$$$$$$$$$         $$$$$$           $$$$$$$$    $$$$$$$$$$
 $$$$$$$$$$$$$$$
       $$$$$$$$$$$$$$$$$$       $$$$$$$           $$$$$$    $$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$ $$$           $$$$    $$$$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$  $$$          $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$   $$$         $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$    $$$        $$$$         $$$$ $$$$
      $$$$     $$     $$$$      $$$$     $$$       $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$      $$$      $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$       $$$     $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$        $$$    $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$         $$$   $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$          $$$  $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$           $$$ $$$$    $$$$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$$$
     $$$$$$          $$$$$$    $$$$$$           $$$$$$$     $$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$
    $$$$$$$$        $$$$$$$$  $$$$$$$$           $$$$$$      $$$$$$$$$$
 $$$$$$$$$$$$$$$

                            LETS START OUR JOURNEY...................

*/

#include <bits/stdc++.h>
#define pb push_back
#define pf push_front
#define in insert
#define ff first
#define ss second
#define off return 0
#define mp make_pair
#define MOD 1000000007
#define ll long long int
#define ld long double
#define snll(a) scanf("%lld", &a)
#define snll2(a, b) scanf("%lld %lld", &a, &b)
#define snll3(a, b, c) scanf("%lld %lld %lld", &a, &b, &c)
#define snl(b) scanf("%d", &b)
#define snl2(a, b) scanf("%d %d", &a, &b)
#define snl3(a, b, c) scanf("%d %d %d", &a, &b, &c)
#define ptl(a) printf("%d\n", a)
#define ptll(a) printf("%lld\n", a)
#define ptl2(a, b) printf("%d %d\n", a, b)
#define ptll2(a, b) printf("%lld %lld\n", a, b)
#define gcd(a, b) __gcd(a, b)
#define lcm(a, b) (a * b) / gcd(a, b)
#define pi acos(-1)
#define C continue
#define B break
#define N "\n"
#define al(v) v.begin(), v.end()
#define srt(v) sort(v.begin(), v.end())
#define rev(v) reverse(v.begin(), v.end())
#define loop(i, a, n) for (int i = a; i < n; i++)
#define T                                                                      \
  int n;                                                                       \
  scanf("%d", &n);                                                             \
  for (int i = 0; i < n; i++)
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  string str, s;
  cin >> str;
  for (int i = 0; i < str.length(); i++) {
    if (str[i] == 'B') {
      if (str.length() > 0)
        s.pop_back();
    } else
      s += str[i];
  }
  cout << s << N;
  off;
}
/*                                     *******THE END******** */


Fixed submission:
/*

 ALLAH IS ALMIGHTY              ******BISMILLAHIR RAHMANIR RAHIM.****** ALLAH IS
 ALMIGHTY
                                    ```````````````````````````````
                                        AUTHOR: ANISUR RAHAMAN
                                            BSMRSTU(SHIICT)
                                                ID:017

                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                               !!!!!!!!
                                           ****!!!!!!!!****
                                            **************
                                             ************
                                              **********
                                               ********
                                                ******
                                                 ****
                                                  **

 _____________________________________________________________________________________________________
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
 `````````````````````````````````````````````````````````````````````````````````````````````````````
 ****** ******
 ***** *****
 **** ****
 *** ***
 ** **
 * *
         $$$$$$$$$$$$$$         $$$$$$           $$$$$$$$    $$$$$$$$$$
 $$$$$$$$$$$$$$$
       $$$$$$$$$$$$$$$$$$       $$$$$$$           $$$$$$    $$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$ $$$           $$$$    $$$$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$  $$$          $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$   $$$         $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$    $$$        $$$$         $$$$ $$$$
      $$$$     $$     $$$$      $$$$     $$$       $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$      $$$      $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$      $$$$       $$$     $$$$         $$$$
 $$$$$$$$$$$$$$$$$$
      $$$$            $$$$      $$$$        $$$    $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$         $$$   $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$          $$$  $$$$         $$$$ $$$$
      $$$$            $$$$      $$$$           $$$ $$$$    $$$$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$$$
     $$$$$$          $$$$$$    $$$$$$           $$$$$$$     $$$$$$$$$$$$
 $$$$$$$$$$$$$$$$$
    $$$$$$$$        $$$$$$$$  $$$$$$$$           $$$$$$      $$$$$$$$$$
 $$$$$$$$$$$$$$$

                            LETS START OUR JOURNEY...................

*/

#include <bits/stdc++.h>
#define pb push_back
#define pf push_front
#define in insert
#define ff first
#define ss second
#define off return 0
#define mp make_pair
#define MOD 1000000007
#define ll long long int
#define ld long double
#define snll(a) scanf("%lld", &a)
#define snll2(a, b) scanf("%lld %lld", &a, &b)
#define snll3(a, b, c) scanf("%lld %lld %lld", &a, &b, &c)
#define snl(b) scanf("%d", &b)
#define snl2(a, b) scanf("%d %d", &a, &b)
#define snl3(a, b, c) scanf("%d %d %d", &a, &b, &c)
#define ptl(a) printf("%d\n", a)
#define ptll(a) printf("%lld\n", a)
#define ptl2(a, b) printf("%d %d\n", a, b)
#define ptll2(a, b) printf("%lld %lld\n", a, b)
#define gcd(a, b) __gcd(a, b)
#define lcm(a, b) (a * b) / gcd(a, b)
#define pi acos(-1)
#define C continue
#define B break
#define N "\n"
#define al(v) v.begin(), v.end()
#define srt(v) sort(v.begin(), v.end())
#define rev(v) reverse(v.begin(), v.end())
#define loop(i, a, n) for (int i = a; i < n; i++)
#define T                                                                      \
  int n;                                                                       \
  scanf("%d", &n);                                                             \
  for (int i = 0; i < n; i++)
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cout.tie(0);

  string str, s;
  cin >> str;
  for (int i = 0; i < str.length(); i++) {
    if (str[i] == 'B') {
      if (s.length() > 0)
        s.pop_back();
    } else
      s += str[i];
  }
  cout << s << N;
  off;
}
/*                                     *******THE END******** */