foo.cc: In function 'std::__cxx11::string repeat(std::__cxx11::string)':
foo.cc:8:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<s.size();i++){
                 ~^~~~~~~~~
foo.cc:12:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0;i<temp.size();i++){
                     ~^~~~~~~~~~~~
foo.cc:17:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=0;i<temp.size();i++){
                     ~^~~~~~~~~~~~
foo.cc: In function 'std::__cxx11::string Get_str(int)':
foo.cc:28:74: error: return-statement with no value, in function returning 'std::__cxx11::string {aka std::__cxx11::basic_string<char>}' [-fpermissive]
     if(!((isdigit(begin)&&isdigit(end))||(islower(begin)&&islower(end))))return;
                                                                          ^~~~~~
foo.cc: In function 'int main()':
foo.cc:40:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for(int i=0;i<s.size();i++){
                 ~^~~~~~~~~

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