Language: C++

#include<iostream>
#include<algorithm>
#include<cmath>
#include<vector>
#include<map>
#include<set>
#include<string>
#include<queue>
#include<stack>
#include<iomanip>

#define fast_io ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);

using namespace std;

#define debug(x) cout<<(#x)<<"="<<(x)<<", ";
#define debug1(x) debug(x) cout<<'\n';
#define debug2(x1, x2) debug(x1) debug(x2) cout<<'\n';
#define debug3(x1, x2, x3) debug(x1) debug(x2) debug(x3) cout<<'\n';
#define debug4(x1, x2, x3, x4) debug(x1) debug(x2) debug(x3) debug(x4) cout<<'\n';
#define debug5(x1, x2, x3, x4, x5) debug(x1) debug(x2) debug(x3) debug(x4) debug(x5) cout<<'\n';

#define fcout cout<<fixed<<setprecision(15)
#define repp(i,j,k) for(int i=j; i<k; ++i)
#define rep(i,j) repp(i,0,j)
#define rrep(i,j,k) for(int i=j; i>=k; --i)
#define all(x) (x).begin(), (x).end()
#define sort(x) sort(x.begin(), x.end())
#define lb(x,val) lower_bound(x.begin(), x.end(), val)
#define ub(x,val) upper_bound(x.begin(), x.end(), val)
#define pb(x) emplace_back(x);

typedef long long int lli;
typedef pair<int,int> pii;
typedef pair<lli,lli> pll;
typedef vector<int> vi;
typedef vector<lli> vl;
typedef tuple<int, int, lli> tiil;

//const lli M=998244353ll;
const lli M=1000000007ll;
const double D=1.0;

int main(){
	fast_io
	string s; cin>>s;
	reverse(all(s));
	int b=0;
	vector<string> p;
	rep(i,s.size()){
		if(s[i]=='B') b++;
		else if(b>0) b--;
		else p.pb(s[i]);
	}
	reverse(all(p));
	rep(i,p.size()){
		cout<<p[i];
	}
	return 0;
}

Compiler output:
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/c++allocator.h:33,
                 from /usr/include/c++/14/bits/allocator.h:46,
                 from /usr/include/c++/14/string:43,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:1:
/usr/include/c++/14/bits/new_allocator.h: In instantiation of 'void std::__new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>]':
/usr/include/c++/14/bits/alloc_traits.h:575:17:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; allocator_type = std::allocator<std::__cxx11::basic_string<char> >]'
  575 |           __a.construct(__p, std::forward<_Args>(__args)...);
      |           ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:117:30:   required from 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; reference = std::__cxx11::basic_string<char>&]'
  117 |             _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
      |             ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  118 |                                      std::forward<_Args>(__args)...);
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:53:10:   required from here
   31 | #define pb(x) emplace_back(x);
      |                           ^
/usr/include/c++/14/bits/new_allocator.h:191:11: error: no matching function for call to 'std::__cxx11::basic_string<char>::basic_string(char&)'
  191 |         { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:54:
/usr/include/c++/14/bits/basic_string.h:800:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  800 |         basic_string(const _Tp& __t, const _Alloc& __a = _Alloc())
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:800:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/move.h:37,
                 from /usr/include/c++/14/bits/exception_ptr.h:41,
                 from /usr/include/c++/14/exception:166,
                 from /usr/include/c++/14/ios:41:
/usr/include/c++/14/type_traits: In substitution of 'template<bool _Cond, class _Tp> using std::enable_if_t = typename std::enable_if::type [with bool _Cond = false; _Tp = void]':
/usr/include/c++/14/bits/basic_string.h:149:8:   required by substitution of 'template<class _CharT, class _Traits, class _Alloc> template<class _Tp, class _Res> using std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv = std::enable_if_t<((bool)std::__and_<std::is_convertible<const _Tp&, std::basic_string_view<_CharT, _Traits> >, std::__not_<std::is_convertible<const _Tp*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>*> >, std::__not_<std::is_convertible<const _Tp&, const _CharT*> > >::value), _Res> [with _Tp = char; _Res = void; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  149 |         using _If_sv = enable_if_t<
      |               ^~~~~~
/usr/include/c++/14/bits/basic_string.h:797:30:   required from 'void std::__new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>]'
  797 |       template<typename _Tp, typename = _If_sv<_Tp, void>>
      |                              ^~~~~~~~
/usr/include/c++/14/bits/alloc_traits.h:575:17:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; allocator_type = std::allocator<std::__cxx11::basic_string<char> >]'
  575 |           __a.construct(__p, std::forward<_Args>(__args)...);
      |           ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:117:30:   required from 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; reference = std::__cxx11::basic_string<char>&]'
  117 |             _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
      |             ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  118 |                                      std::forward<_Args>(__args)...);
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:53:10:   required from here
   31 | #define pb(x) emplace_back(x);
      |                           ^
/usr/include/c++/14/type_traits:2711:11: error: no type named 'type' in 'struct std::enable_if<false, void>'
 2711 |     using enable_if_t = typename enable_if<_Cond, _Tp>::type;
      |           ^~~~~~~~~~~
/usr/include/c++/14/bits/new_allocator.h: In instantiation of 'void std::__new_allocator<_Tp>::construct(_Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>]':
/usr/include/c++/14/bits/alloc_traits.h:575:17:   required from 'static void std::allocator_traits<std::allocator<_CharT> >::construct(allocator_type&, _Up*, _Args&& ...) [with _Up = std::__cxx11::basic_string<char>; _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; allocator_type = std::allocator<std::__cxx11::basic_string<char> >]'
  575 |           __a.construct(__p, std::forward<_Args>(__args)...);
      |           ~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/vector.tcc:117:30:   required from 'std::vector<_Tp, _Alloc>::reference std::vector<_Tp, _Alloc>::emplace_back(_Args&& ...) [with _Args = {char&}; _Tp = std::__cxx11::basic_string<char>; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; reference = std::__cxx11::basic_string<char>&]'
  117 |             _Alloc_traits::construct(this->_M_impl, this->_M_impl._M_finish,
      |             ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  118 |                                      std::forward<_Args>(__args)...);
      |                                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:53:10:   required from here
   31 | #define pb(x) emplace_back(x);
      |                           ^
/usr/include/c++/14/bits/basic_string.h:788:9: note: candidate: 'template<class _Tp, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _Tp&, size_type, size_type, const _Alloc&) [with <template-parameter-2-2> = _Tp; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  788 |         basic_string(const _Tp& __t, size_type __pos, size_type __n,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:788:9: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:765:9: note: candidate: 'template<class _InputIterator, class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(_InputIterator, _InputIterator, const _Alloc&) [with <template-parameter-2-2> = _InputIterator; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  765 |         basic_string(_InputIterator __beg, _InputIterator __end,
      |         ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:765:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:669:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(size_type, _CharT, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  669 |       basic_string(size_type __n, _CharT __c, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:669:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:646:7: note: candidate: 'template<class> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const _CharT*, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  646 |       basic_string(const _CharT* __s, const _Alloc& __a = _Alloc())
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:646:7: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/new_allocator.h:191:11: note:   cannot convert 'std::forward<char&>((* & __args#0))' (type 'char') to type 'const char*'
  191 |         { ::new((void *)__p) _Up(std::forward<_Args>(__args)...); }
      |           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:721:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&&, const _Alloc&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  721 |       basic_string(basic_string&& __str, const _Alloc& __a)
      |       ^~~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:721:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/basic_string.h:716:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::basic_string(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&, const _Alloc&) [with _CharT = char; _Tr