foo.cc: In function 'std::string Concat(std::string, std::string)':
foo.cc:7:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    7 |     for(int i=0;i<s1.size()-1;i++){
      |                 ~^~~~~~~~~~~~
foo.cc:10:18: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   10 |     for(int i=1;i<s2.size();i++){
      |                 ~^~~~~~~~~~