Language: C++

#include<bits/stdc++.h>
inline int getint() {
    register char ch;
    while(!isdigit(ch=getchar()));
    register int x=ch^'0';
    while(isdigit(ch=getchar())) x=(((x<<2)+x)<<1)+(ch^'0');
    return x;
}
int main()
{
    for(register int i=getint();i;i--) set.insert("a");
    for(register int i=getint();i;i--) set.insert("b");
    for(register int i=getint();i;i--) set.insert("c");
    while(set.size()>1) {
        std::string s=*set.begin(),t=*set.rbegin();
        set.erase(set.lower_bound(s));
        set.erase(set.lower_bound(t));
        set.insert(s+t);
    }
    printf("%s\n",(*set.begin()).c_str());
    return 0;
} 

Compiler output:
a.cc: In function 'int getint()':
a.cc:3:19: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    3 |     register char ch;
      |                   ^~
a.cc:5:18: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
    5 |     register int x=ch^'0';
      |                  ^
a.cc: In function 'int main()':
a.cc:11:22: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
   11 |     for(register int i=getint();i;i--) set.insert("a");
      |                      ^
a.cc:11:40: error: 'set' was not declared in this scope
   11 |     for(register int i=getint();i;i--) set.insert("a");
      |                                        ^~~
a.cc:11:40: note: suggested alternatives:
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:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~
a.cc:12:22: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
   12 |     for(register int i=getint();i;i--) set.insert("b");
      |                      ^
a.cc:12:40: error: 'set' was not declared in this scope
   12 |     for(register int i=getint();i;i--) set.insert("b");
      |                                        ^~~
a.cc:12:40: note: suggested alternatives:
/usr/include/c++/14/bits/stl_set.h:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~
a.cc:13:22: warning: ISO C++17 does not allow 'register' storage class specifier [-Wregister]
   13 |     for(register int i=getint();i;i--) set.insert("c");
      |                      ^
a.cc:13:40: error: 'set' was not declared in this scope
   13 |     for(register int i=getint();i;i--) set.insert("c");
      |                                        ^~~
a.cc:13:40: note: suggested alternatives:
/usr/include/c++/14/bits/stl_set.h:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~
a.cc:14:11: error: 'set' was not declared in this scope
   14 |     while(set.size()>1) {
      |           ^~~
a.cc:14:11: note: suggested alternatives:
/usr/include/c++/14/bits/stl_set.h:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~
a.cc:17:35: error: 't' was not declared in this scope
   17 |         set.erase(set.lower_bound(t));
      |                                   ^
a.cc:20:21: error: 'set' was not declared in this scope
   20 |     printf("%s\n",(*set.begin()).c_str());
      |                     ^~~
a.cc:20:21: note: suggested alternatives:
/usr/include/c++/14/bits/stl_set.h:96:11: note:   'std::set'
   96 |     class set
      |           ^~~
/usr/include/c++/14/set:87:13: note:   'std::pmr::set'
   87 |       using set = std::set<_Key, _Cmp, polymorphic_allocator<_Key>>;
      |             ^~~