Language: C++

#include <bits/stdc++.h>
using namespace std;

int main(){
  string s;
  cin >> s;
  set<char> used;
  for (char c:s) used.insert(c);
  sort(used.begin(),used.end());
  for (char i = 'a'; i <='z'; i++) {
    if (used.count()) continue;
    else{
      cout << i << endl;
      break;
    }
  }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:11:19: error: no matching function for call to 'std::set<char>::count()'
   11 |     if (used.count()) continue;
      |         ~~~~~~~~~~^~
In file included from /usr/include/c++/14/set:63,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:158,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_set.h:756:9: note: candidate: 'template<class _Kt> decltype (((const std::set<_Key, _Compare, _Alloc>*)this)->std::set<_Key, _Compare, _Alloc>::_M_t._M_count_tr(__x)) std::set<_Key, _Compare, _Alloc>::count(const _Kt&) const [with _Key = char; _Compare = std::less<char>; _Alloc = std::allocator<char>]'
  756 |         count(const _Kt& __x) const
      |         ^~~~~
/usr/include/c++/14/bits/stl_set.h:756:9: note:   candidate expects 1 argument, 0 provided
/usr/include/c++/14/bits/stl_set.h:750:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::size_type std::set<_Key, _Compare, _Alloc>::count(const key_type&) const [with _Key = char; _Compare = std::less<char>; _Alloc = std::allocator<char>; size_type = long unsigned int; key_type = char]'
  750 |       count(const key_type& __x) const
      |       ^~~~~
/usr/include/c++/14/bits/stl_set.h:750:7: note:   candidate expects 1 argument, 0 provided
In file included from /usr/include/c++/14/algorithm:61,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/bits/stl_algo.h: In instantiation of 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = _Rb_tree_const_iterator<char>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]':
/usr/include/c++/14/bits/stl_algo.h:4772:18:   required from 'void std::sort(_RAIter, _RAIter) [with _RAIter = _Rb_tree_const_iterator<char>]'
 4772 |       std::__sort(__first, __last, __gnu_cxx::__ops::__iter_less_iter());
      |       ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:9:7:   required from here
    9 |   sort(used.begin(),used.end());
      |   ~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algo.h:1906:50: error: no match for 'operator-' (operand types are 'std::_Rb_tree_const_iterator<char>' and 'std::_Rb_tree_const_iterator<char>')
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
In file included from /usr/include/c++/14/bits/stl_algobase.h:67,
                 from /usr/include/c++/14/algorithm:60:
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::reverse_iterator<_Iterator>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::move_iterator<_IteratorL>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
In file included from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:166:
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_Constant, std::_ValArray, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const typename valarray<_Tp>::value_type&, const valarray<_Tp>&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::valarray<_Tp>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note: candidate: 'template<class _Tp> std::_Expr<std::__detail::_BinClos<std::__minus, std::_ValArray, std::_Constant, _Tp, _Tp>, typename std::__fun<std::__minus, _Tp>::result_type> std::operator-(const valarray<_Tp>&, const typename valarray<_Tp>::value_type&)'
 1197 | _DEFINE_BINARY_OPERATOR(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::valarray<_Tp>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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(-, __minus)
      | ^~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/valarray:1197:1: note:   template argument deduction/substitution failed:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::valarray<_Tp>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
In file included from /usr/include/c++/14/valarray:605:
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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:
/usr/include/c++/14/bits/stl_algo.h:1906:50: note:   'std::_Rb_tree_const_iterator<char>' is not derived from 'const std::_Expr<_Dom1, typename _Dom1::value_type>'
 1906 |                                 std::__lg(__last - __first) * 2,
      |                                           ~~~~~~~^~~~~~~~~
/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, typena