Language: C++

#include<iostream>
using namespace std;
long long key[1000000];
int fro = 0;
int end = 0;
void push(long long a) {
  key[end] = a;
  end ++;
}
long long pop() {
  fro ++;
  return key[fro - 1];
}
int main () {
  int K;
  cin >> K;
  for (int i = 0; i < 9; i ++) push(i + 1);
  long long ans = 0;
  for (int i = 0; i < K; i ++) {
    ans = pop();
    long long ku = ans * 10;
    long long ma = ans % 10;
    if (ma != 0) push(ku + ma - 1);
    push(ku + ma);
    if (ma != 9) push(ku + ma + 1);
  }
  cout << ans << endl;
}


Compiler output:
a.cc: In function 'void push(long long int)':
a.cc:7:7: error: reference to 'end' is ambiguous
    7 |   key[end] = a;
      |       ^~~
In file included from /usr/include/c++/14/string:53,
                 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/range_access.h:116:37: note: candidates are: 'template<class _Tp> const _Tp* std::end(const valarray<_Tp>&)'
  116 |   template<typename _Tp> const _Tp* end(const valarray<_Tp>&) noexcept;
      |                                     ^~~
/usr/include/c++/14/bits/range_access.h:115:31: note:                 'template<class _Tp> _Tp* std::end(valarray<_Tp>&)'
  115 |   template<typename _Tp> _Tp* end(valarray<_Tp>&) noexcept;
      |                               ^~~
/usr/include/c++/14/bits/range_access.h:106:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::end(_Tp (&)[_Nm])'
  106 |     end(_Tp (&__arr)[_Nm]) noexcept
      |     ^~~
/usr/include/c++/14/bits/range_access.h:85:5: note:                 'template<class _Container> constexpr decltype (__cont.end()) std::end(const _Container&)'
   85 |     end(const _Container& __cont) -> decltype(__cont.end())
      |     ^~~
/usr/include/c++/14/bits/range_access.h:74:5: note:                 'template<class _Container> constexpr decltype (__cont.end()) std::end(_Container&)'
   74 |     end(_Container& __cont) -> decltype(__cont.end())
      |     ^~~
In file included from /usr/include/c++/14/bits/range_access.h:36:
/usr/include/c++/14/initializer_list:99:5: note:                 'template<class _Tp> constexpr const _Tp* std::end(initializer_list<_Tp>)'
   99 |     end(initializer_list<_Tp> __ils) noexcept
      |     ^~~
a.cc:5:5: note:                 'int end'
    5 | int end = 0;
      |     ^~~
a.cc:8:3: error: reference to 'end' is ambiguous
    8 |   end ++;
      |   ^~~
/usr/include/c++/14/bits/range_access.h:116:37: note: candidates are: 'template<class _Tp> const _Tp* std::end(const valarray<_Tp>&)'
  116 |   template<typename _Tp> const _Tp* end(const valarray<_Tp>&) noexcept;
      |                                     ^~~
/usr/include/c++/14/bits/range_access.h:115:31: note:                 'template<class _Tp> _Tp* std::end(valarray<_Tp>&)'
  115 |   template<typename _Tp> _Tp* end(valarray<_Tp>&) noexcept;
      |                               ^~~
/usr/include/c++/14/bits/range_access.h:106:5: note:                 'template<class _Tp, long unsigned int _Nm> constexpr _Tp* std::end(_Tp (&)[_Nm])'
  106 |     end(_Tp (&__arr)[_Nm]) noexcept
      |     ^~~
/usr/include/c++/14/bits/range_access.h:85:5: note:                 'template<class _Container> constexpr decltype (__cont.end()) std::end(const _Container&)'
   85 |     end(const _Container& __cont) -> decltype(__cont.end())
      |     ^~~
/usr/include/c++/14/bits/range_access.h:74:5: note:                 'template<class _Container> constexpr decltype (__cont.end()) std::end(_Container&)'
   74 |     end(_Container& __cont) -> decltype(__cont.end())
      |     ^~~
/usr/include/c++/14/initializer_list:99:5: note:                 'template<class _Tp> constexpr const _Tp* std::end(initializer_list<_Tp>)'
   99 |     end(initializer_list<_Tp> __ils) noexcept
      |     ^~~
a.cc:5:5: note:                 'int end'
    5 | int end = 0;
      |     ^~~