Language: C++

#include<bits/stdc++.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define pb(a) push_back(a)
#define all(a) (a).begin(), (a).end()
#define debug(x) cout << "debug " << x << endl;
using namespace std;

string solve(string s){
    int count = 1;
    string r;
    rep(i,s.size() - 1){
        if(s[i] == s[i + 1]){
            count++;
        }else{
            r+=
            cout << count << s[i];
            count = 1;
        }
    }
    return s;
}

int main(){
    int n;
    string s;
    while(cin >> n,n){
        cin >> s;
        rep(i,n){
            s = solve(s);
        }
        cout << s << endl;
    }
}

Compiler output:
a.cc: In function 'std::string solve(std::string)':
a.cc:16:14: error: no match for 'operator+=' (operand types are 'std::string' {aka 'std::__cxx11::basic_string<char>'} and 'std::basic_ostream<char>')
   16 |             r+=
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:1:
/usr/include/c++/14/bits/basic_string.h:1424:9: note: candidate: 'template<class _Tp> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1424 |         operator+=(const _Tp& __svt)
      |         ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1424:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/stl_pair.h:60,
                 from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/type_traits: In substitution of 'template<bool _Cond, class _Tp> using std::enable_if_t = typename std::enable_if::type [with bool _Cond = false; _Tp = std::__cxx11::basic_string<char>&]':
/usr/include/c++/14/bits/basic_string.h:149:8:   required by substitution of 'template<class _CharT, class _Traits, class _Alloc> template<class _Tp, class _Res> using std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv = std::enable_if_t<((bool)std::__and_<std::is_convertible<const _Tp&, std::basic_string_view<_CharT, _Traits> >, std::__not_<std::is_convertible<const _Tp*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>*> >, std::__not_<std::is_convertible<const _Tp&, const _CharT*> > >::value), _Res> [with _Tp = std::basic_ostream<char>; _Res = std::__cxx11::basic_string<char>&; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  149 |         using _If_sv = enable_if_t<
      |               ^~~~~~
/usr/include/c++/14/bits/basic_string.h:1424:2:   required by substitution of 'template<class _Tp> std::__cxx11::basic_string<char>::_If_sv<_Tp, std::__cxx11::basic_string<char>&> std::__cxx11::basic_string<char>::operator+=(const _Tp&) [with _Tp = std::basic_ostream<char>]'
 1424 |         operator+=(const _Tp& __svt)
      |         ^~~~~~~~
a.cc:17:33:   required from here
   17 |             cout << count << s[i];
      |                                 ^
/usr/include/c++/14/type_traits:2711:11: error: no type named 'type' in 'struct std::enable_if<false, std::__cxx11::basic_string<char>&>'
 2711 |     using enable_if_t = typename enable_if<_Cond, _Tp>::type;
      |           ^~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1377:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1377 |       operator+=(const basic_string& __str)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1377:38: note:   no known conversion for argument 1 from 'std::basic_ostream<char>' to 'const std::__cxx11::basic_string<char>&'
 1377 |       operator+=(const basic_string& __str)
      |                  ~~~~~~~~~~~~~~~~~~~~^~~~~
/usr/include/c++/14/bits/basic_string.h:1387:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1387 |       operator+=(const _CharT* __s)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1387:32: note:   no known conversion for argument 1 from 'std::basic_ostream<char>' to 'const char*'
 1387 |       operator+=(const _CharT* __s)
      |                  ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1397:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(_CharT) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1397 |       operator+=(_CharT __c)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1397:25: note:   no known conversion for argument 1 from 'std::basic_ostream<char>' to 'char'
 1397 |       operator+=(_CharT __c)
      |                  ~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1411:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(std::initializer_list<_Tp>) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1411 |       operator+=(initializer_list<_CharT> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1411:43: note:   no known conversion for argument 1 from 'std::basic_ostream<char>' to 'std::initializer_list<char>'
 1411 |       operator+=(initializer_list<_CharT> __l)
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~^~~