Language: C++

#include <algorithm>
#include <iostream>
using namespace std;

int main(){
	string S, A='qwertyuiopasdfghjklzxcvbnm';
	cin >> S;
	for(int i=0; i<S.size(); i++){
		for(int j=0; j<A.size(); j++){
			if(S[i]==A[j]){
				A[j] = 'A';
			}
		}
	}
	if(min(A)=='A'){
		cout << "None" << endl;
	} else {
		cout << min(A) << endl;
	}
}

Compiler output:
a.cc:6:21: warning: multi-character literal with 26 characters exceeds 'int' size of 4 bytes
    6 |         string S, A='qwertyuiopasdfghjklzxcvbnm';
      |                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc: In function 'int main()':
a.cc:6:21: error: conversion from 'int' to non-scalar type 'std::string' {aka 'std::__cxx11::basic_string<char>'} requested
a.cc:15:15: error: no matching function for call to 'min(std::string&)'
   15 |         if(min(A)=='A'){
      |            ~~~^~~
In file included from /usr/include/c++/14/algorithm:60,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 1 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   template argument deduction/substitution failed:
a.cc:15:15: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::initializer_list<_Tp>'
   15 |         if(min(A)=='A'){
      |            ~~~^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   candidate expects 2 arguments, 1 provided
a.cc:18:28: error: no matching function for call to 'min(std::string&)'
   18 |                 cout << min(A) << endl;
      |                         ~~~^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   candidate expects 2 arguments, 1 provided
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 1 provided
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   template argument deduction/substitution failed:
a.cc:18:28: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::initializer_list<_Tp>'
   18 |                 cout << min(A) << endl;
      |                         ~~~^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   candidate expects 2 arguments, 1 provided