Language: C++

#include <bits/stdc++.h>
#define _GLIBCXX_DEBUG
using namespace std;
using ll = long long;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
int main() {
  string A, B, C;
  cin >> A >> B >> C;
  int Al = A.size();
  int Bl = B.size();
  int Cl = C.size();
  int Ac = Bc = Cc = 0;
  int a = b = c = 0;
  a++;
  rep(i, A + B + C - 3) {
    if(a == 1) {
      if(A.at(Ac) == 'a') {
        Ac++;
      }
      if(A.at(Ac) == 'b') {
        Bc++;
        a--;
        b++;
      }
      if(A.at(Ac) == 'c') {
        Cc++;
        a--;
        c++;
      }
    }
    if(b == 1) {
      if(A.at(Bc) == 'a') {
        Bc++;
        b--;
        a++;
      }
      if(A.at(Bc) == 'b') {
        Bc++;
      }
      if(A.at(Bc) == 'c') {
        Bc++;
        b--;
        c++;
      }
    }
    if(c == 1) {
      if(A.at(Cc) == 'a') {
        Cc++;
        c--;
        b++;
      }
      if(A.at(Cc) == 'b') {
        Cc++;
        c--;
        b++;
      }
      if(A.at(Cc) == 'c') {
        Cc++;
      }
    }
    if(Ac == Al - 1 || Bc == Bl - 1 || Cc == Cl - 1) {
      break;
    }
  }
  if(a == 1) {
    cout << "A" << endl;
  }
  if(b == 1) {
    cout << "B" << endl;
  }
  if(c == 1) {
    cout << "C" << endl;
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:13:12: error: 'Bc' was not declared in this scope; did you mean 'Ac'?
   13 |   int Ac = Bc = Cc = 0;
      |            ^~
      |            Ac
a.cc:13:17: error: 'Cc' was not declared in this scope; did you mean 'Ac'?
   13 |   int Ac = Bc = Cc = 0;
      |                 ^~
      |                 Ac
a.cc:14:11: error: 'b' was not declared in this scope
   14 |   int a = b = c = 0;
      |           ^
a.cc:14:15: error: 'c' was not declared in this scope
   14 |   int a = b = c = 0;
      |               ^
a.cc:16:20: error: no match for 'operator-' (operand types are 'std::__cxx11::basic_string<char>' and 'int')
   16 |   rep(i, A + B + C - 3) {
      |          ~~~~~~~~~ ^ ~
      |                |     |
      |                |     int
      |                std::__cxx11::basic_string<char>
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_iterator.h:618:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__y.base() - __x.base())) std::operator-(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  618 |     operator-(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:618:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr decltype ((__x.base() - __y.base())) std::operator-(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1790 |     operator-(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1790:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::move_iterator<_IteratorL>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
In file included from /usr/include/c++/14/ccomplex:39,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:127:
/usr/include/c++/14/complex:370:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const complex<_Tp>&)'
  370 |     operator-(const complex<_Tp>& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:370:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::complex<_Tp>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/complex:379:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&, const _Tp&)'
  379 |     operator-(const complex<_Tp>& __x, const _Tp& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:379:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::complex<_Tp>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/complex:388:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const _Tp&, const complex<_Tp>&)'
  388 |     operator-(const _Tp& __x, const complex<_Tp>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/complex:388:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   mismatched types 'const std::complex<_Tp>' and 'int'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/complex:465:5: note: candidate: 'template<class _Tp> std::complex<_Tp> std::operator-(const complex<_Tp>&)'
  465 |     operator-(const complex<_Tp>& __x)
      |     ^~~~~~~~
/usr/include/c++/14/complex:465:5: note:   candidate expects 1 argument, 2 provided
In file included from /usr/include/c++/14/valarray:605,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom1, class _Dom2> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Expr, _Dom1, _Dom2>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const _Expr<_Dom2, typename _Dom2::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_Constant, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const typename _Dom::value_type&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const typename _Dom::value_type&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Expr, std::_ValArray, _Dom, typename _Dom::value_type>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const _Expr<_Dom1, typename _Dom1::value_type>&, const valarray<typename _Dom::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   'std::__cxx11::basic_string<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/bits/valarray_after.h:406:5: note: candidate: 'template<class _Dom> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Expr, typename _Dom::value_type, _Dom>, typename std::__fun<std::__minus, typename _Dom1::value_type>::result_type> std::operator-(const valarray<typename _Dom::value_type>&, const _Expr<_Dom1, typename _Dom1::value_type>&)'
  406 |     _DEFINE_EXPR_BINARY_OPERATOR(-, struct std::__minus)
      |     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/valarray_after.h:406:5: note:   template argument deduction/substitution failed:
a.cc:16:22: note:   mismatched types 'const std::_Expr<_Dom1, typename _Dom1::value_type>' and 'int'
   16 |   rep(i, A + B + C - 3) {
      |                      ^
a.cc:5:45: note: in definition of macro 'rep'
    5 | #define rep(i, n) for (int i = 0; i < (int)(n); i++)
      |                                             ^
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __m