foo.cc: In function 'int main()':
foo.cc:12:60: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
  if (s1.length() < s2.length()||s1.length() == s2.length() && s1<s2){
                                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~
foo.cc:20:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while (i < s2.length()){
         ~~^~~~~~~~~~~~~
foo.cc:29:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  while (i < s1.length()){
         ~~^~~~~~~~~~~~~
foo.cc:40:18: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for(int i = 0;i < ans.length();i++){
                ~~^~~~~~~~~~~~~~

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