Language: C++

#include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
 
using namespace std;
 
#define rep(i,n) for(int i=0; i<(n); i++)
#define repc(i,s,e) for(int i=(s); i<(e); i++)
#define pb(n) push_back((n))
#define mp(n,m) make_pair((n),(m))
#define all(r) r.begin(),r.end()
#define fi first
#define se second
 
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef vector<ll> vl;
typedef vector<vl> vll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
 
const int INF = 10000000;
const int mod = 1e9 + 7;


int main(){
	string s, t;
	set<string> st;
	s = "a";
	st.insert(s);
	cout<<"?"<<s<<endl;
	while(1){
		cin>>t;
		if(s[s.size() - 1] != t[0] || st.count(t)){
			cout<<"!OUT"<<endl;
			break;
		}
		st.insert(t);
		s = t[t.size()-1] + "a";
		while(st.count(s) && s < 10){
			s+="a";
		}

		for(int i = 1; i < 26 && st.count(s); i++){
			s[1] += 1;
		}

		for(int i = 1; i < 26 && st.count(s); i++){
			s[2] += 1;
		}		

		cout<<"?"<<s<<endl;
	}
}

Compiler output:
a.cc: In function 'int main()':
a.cc:53:40: error: no match for 'operator<' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'int')
   53 |                 while(st.count(s) && s < 10){
      |                                      ~ ^ ~~
      |                                      |   |
      |                                      |   int
      |                                      std::string {aka std::__cxx11::basic_string<char>}
In file included from /usr/include/c++/14/string:48,
                 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/stl_iterator.h:448:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_Iterator>&)'
  448 |     operator<(const reverse_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:448:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const reverse_iterator<_Iterator>&, const reverse_iterator<_IteratorR>&)'
  493 |     operator<(const reverse_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:493:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::reverse_iterator<_Iterator>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note: candidate: 'template<class _IteratorL, class _IteratorR> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorR>&)'
 1694 |     operator<(const move_iterator<_IteratorL>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1694:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note: candidate: 'template<class _Iterator> constexpr bool std::operator<(const move_iterator<_IteratorL>&, const move_iterator<_IteratorL>&)'
 1760 |     operator<(const move_iterator<_Iterator>& __x,
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_iterator.h:1760:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::move_iterator<_IteratorL>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/string:51:
/usr/include/c++/14/bits/stl_pair.h:1045:5: note: candidate: 'template<class _T1, class _T2> constexpr bool std::operator<(const pair<_T1, _T2>&, const pair<_T1, _T2>&)'
 1045 |     operator<(const pair<_T1, _T2>& __x, const pair<_T1, _T2>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_pair.h:1045:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::pair<_T1, _T2>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/bits/basic_string.h:47,
                 from /usr/include/c++/14/string:54:
/usr/include/c++/14/string_view:673:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, basic_string_view<_CharT, _Traits>)'
  673 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:673:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/string_view:680:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(basic_string_view<_CharT, _Traits>, __type_identity_t<basic_string_view<_CharT, _Traits> >)'
  680 |     operator< (basic_string_view<_CharT, _Traits> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:680:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::__cxx11::basic_string<char>' is not derived from 'std::basic_string_view<_CharT, _Traits>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/string_view:688:5: note: candidate: 'template<class _CharT, class _Traits> constexpr bool std::operator<(__type_identity_t<basic_string_view<_CharT, _Traits> >, basic_string_view<_CharT, _Traits>)'
  688 |     operator< (__type_identity_t<basic_string_view<_CharT, _Traits>> __x,
      |     ^~~~~~~~
/usr/include/c++/14/string_view:688:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   mismatched types 'std::basic_string_view<_CharT, _Traits>' and 'int'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/basic_string.h:3874:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3874 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3874:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   mismatched types 'const std::__cxx11::basic_string<_CharT, _Traits, _Allocator>' and 'int'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/basic_string.h:3888:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const __cxx11::basic_string<_CharT, _Traits, _Allocator>&, const _CharT*)'
 3888 |     operator<(const basic_string<_CharT, _Traits, _Alloc>& __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3888:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   mismatched types 'const _CharT*' and 'int'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
/usr/include/c++/14/bits/basic_string.h:3901:5: note: candidate: 'template<class _CharT, class _Traits, class _Alloc> bool std::operator<(const _CharT*, const __cxx11::basic_string<_CharT, _Traits, _Allocator>&)'
 3901 |     operator<(const _CharT* __lhs,
      |     ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:3901:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   mismatched types 'const _CharT*' and 'std::__cxx11::basic_string<char>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/bits/memory_resource.h:47,
                 from /usr/include/c++/14/string:68:
/usr/include/c++/14/tuple:2600:5: note: candidate: 'template<class ... _TElements, class ... _UElements> constexpr bool std::operator<(const tuple<_UTypes ...>&, const tuple<_Elements ...>&)'
 2600 |     operator<(const tuple<_TElements...>& __t,
      |     ^~~~~~~~
/usr/include/c++/14/tuple:2600:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::tuple<_UTypes ...>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/vector:66,
                 from a.cc:5:
/usr/include/c++/14/bits/stl_vector.h:2089:5: note: candidate: 'template<class _Tp, class _Alloc> bool std::operator<(const vector<_Tp, _Alloc>&, const vector<_Tp, _Alloc>&)'
 2089 |     operator<(const vector<_Tp, _Alloc>& __x, const vector<_Tp, _Alloc>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_vector.h:2089:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::vector<_Tp, _Alloc>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/deque:66,
                 from /usr/include/c++/14/stack:62,
                 from a.cc:7:
/usr/include/c++/14/bits/stl_deque.h:2334:5: note: candidate: 'template<class _Tp, class _Alloc> bool std::operator<(const deque<_Tp, _Alloc>&, const deque<_Tp, _Alloc>&)'
 2334 |     operator<(const deque<_Tp, _Alloc>& __x, const deque<_Tp, _Alloc>& __y)
      |     ^~~~~~~~
/usr/include/c++/14/bits/stl_deque.h:2334:5: note:   template argument deduction/substitution failed:
a.cc:53:42: note:   'std::string' {aka 'std::__cxx11::basic_string<char>'} is not derived from 'const std::deque<_Tp, _Alloc>'
   53 |                 while(st.count(s) && s < 10){
      |                                          ^~
In file included from /usr/include/c++/14/stack:63:
/usr/include/c++/14/bits/stl_stack.h:373:5: note: candidate: 'template<class _Tp, class _Seq> bool std::operator<(const stack<_Tp, _Seq>&, const stack<_Tp, _Seq>&)'