foo.cc: In function 'int main()':
foo.cc:6:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size(); i++) maxn = min(maxn, s[i]-'0');
                  ~~^~~~~~~~~~
foo.cc:12:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.size(); i++) {
                  ~~^~~~~~~~~~

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