Language: C++

#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i=0;i<(int)(n);i++)

int main(){
    string s;
    cin>>s;
    set<string>seet;
    rep(i,s.size()){
        seet.insert(s[i]);
    }
    char unit[]={'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    string abc(unit,end(unit));
    char MIN = "z";
    int cnt=0;
    for(int i = 0; i < abc.size();i++){
        decltype(seet)::iterator it = seet.find(abc[i]);
        if(it != seet.end()){//見つかった場合
            cnt++; //cnt=abc.size()ならNone
        }else{
            MIN = min(MIN,abc[i]);
        }
    }
    if(cnt==abc.size()){
        cout<<"None"<<endl;
    }else{
        cout<<MIN<<endl;
    }
}

Compiler output:
a.cc: In function 'int main()':
a.cc:10:20: error: no matching function for call to 'std::set<std::__cxx11::basic_string<char> >::insert(__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type&)'
   10 |         seet.insert(s[i]);
      |         ~~~~~~~~~~~^~~~~~
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:568:9: note: candidate: 'template<class _InputIterator> void std::set<_Key, _Compare, _Alloc>::insert(_InputIterator, _InputIterator) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]'
  568 |         insert(_InputIterator __first, _InputIterator __last)
      |         ^~~~~~
/usr/include/c++/14/bits/stl_set.h:568:9: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:511:7: note: candidate: 'std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other = std::allocator<std::__cxx11::basic_string<char> >; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key> = __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::rebind<std::__cxx11::basic_string<char> >; typename _Alloc::value_type = std::__cxx11::basic_string<char>; value_type = std::__cxx11::basic_string<char>]'
  511 |       insert(const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:511:32: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'const std::set<std::__cxx11::basic_string<char> >::value_type&' {aka 'const std::__cxx11::basic_string<char>&'}
  511 |       insert(const value_type& __x)
      |              ~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:520:7: note: candidate: 'std::pair<typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator, bool> std::set<_Key, _Compare, _Alloc>::insert(value_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; typename std::_Rb_tree<_Key, _Key, std::_Identity<_Tp>, _Compare, typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other>::const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key>::other = std::allocator<std::__cxx11::basic_string<char> >; typename __gnu_cxx::__alloc_traits<_Alloc>::rebind<_Key> = __gnu_cxx::__alloc_traits<std::allocator<std::__cxx11::basic_string<char> >, std::__cxx11::basic_string<char> >::rebind<std::__cxx11::basic_string<char> >; typename _Alloc::value_type = std::__cxx11::basic_string<char>; value_type = std::__cxx11::basic_string<char>]'
  520 |       insert(value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:520:27: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::set<std::__cxx11::basic_string<char> >::value_type&&' {aka 'std::__cxx11::basic_string<char>&&'}
  520 |       insert(value_type&& __x)
      |              ~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:548:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, const value_type&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  548 |       insert(const_iterator __position, const value_type& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:548:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:553:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, value_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; value_type = std::__cxx11::basic_string<char>]'
  553 |       insert(const_iterator __position, value_type&& __x)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:553:7: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_set.h:580:7: note: candidate: 'void std::set<_Key, _Compare, _Alloc>::insert(std::initializer_list<_Tp>) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >]'
  580 |       insert(initializer_list<value_type> __l)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:580:43: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::initializer_list<std::__cxx11::basic_string<char> >'
  580 |       insert(initializer_list<value_type> __l)
      |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/stl_set.h:600:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::insert_return_type std::set<_Key, _Compare, _Alloc>::insert(node_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; insert_return_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::insert_return_type; node_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type]'
  600 |       insert(node_type&& __nh)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:600:26: note:   no known conversion for argument 1 from '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'} to 'std::set<std::__cxx11::basic_string<char> >::node_type&&' {aka 'std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type&&'}
  600 |       insert(node_type&& __nh)
      |              ~~~~~~~~~~~~^~~~
/usr/include/c++/14/bits/stl_set.h:605:7: note: candidate: 'std::set<_Key, _Compare, _Alloc>::iterator std::set<_Key, _Compare, _Alloc>::insert(const_iterator, node_type&&) [with _Key = std::__cxx11::basic_string<char>; _Compare = std::less<std::__cxx11::basic_string<char> >; _Alloc = std::allocator<std::__cxx11::basic_string<char> >; iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; const_iterator = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::const_iterator; node_type = std::_Rb_tree<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char>, std::_Identity<std::__cxx11::basic_string<char> >, std::less<std::__cxx11::basic_string<char> >, std::allocator<std::__cxx11::basic_string<char> > >::node_type]'
  605 |       insert(const_iterator __hint, node_type&& __nh)
      |       ^~~~~~
/usr/include/c++/14/bits/stl_set.h:605:7: note:   candidate expects 2 arguments, 1 provided
a.cc:14:16: error: invalid conversion from 'const char*' to 'char' [-fpermissive]
   14 |     char MIN = "z";
      |                ^~~
      |                |
      |