/in/foo.cc: In function 'void Concat(char*, char*, char*)':
/in/foo.cc:10:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(;i<strlen(s1)-1;i++)
       ~^~~~~~~~~~~~~
/in/foo.cc:12:8: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(;j<strlen(s2);j++)
       ~^~~~~~~~~~~