combo.cpp: In function 'std::string guess_sequence(int)':
combo.cpp:15:22: warning: comparison of integer expressions of different signedness: 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   15 |     while(ret.size() < N-1){
      |           ~~~~~~~~~~~^~~~~
combo.cpp:17:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   17 |         if(coins == ret.size())
      |            ~~~~~~^~~~~~~~~~~~~
combo.cpp:19:25: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   19 |         else if(coins-1 == ret.size())
      |                 ~~~~~~~~^~~~~~~~~~~~~