foo.cc: In function 'int main()':
foo.cc:8:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s1.size(); i++) if(s1[i]>='a' && s1[i] <= 'z') cnt1++;
                  ~~^~~~~~~~~~~
foo.cc:9:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s2.size(); i++) if(s2[i]>='a' && s2[i] <= 'z') cnt2++;
                  ~~^~~~~~~~~~~

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