foo.cc: In function 'std::__cxx11::string sub(std::__cxx11::string, std::__cxx11::string)':
foo.cc:38:50: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
  if (a.size() < b.size() || a.size() == b.size() && a < b)
                             ~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。