/in/foo.cc: In function 'int main()':
/in/foo.cc:7:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if(s1.find(s2)!=-1)cout<<s2<<" is substring of "<<s1<<endl;//判断s2是不是s1的子串，-1为不是
     ~~~~~~~~~~~^~~~
/in/foo.cc:8:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  else if(s2.find(s1)!=-1)cout<<s1<<" is substring of "<<s2<<endl;//判断s1是不是s2的子串，-1为不是
          ~~~~~~~~~~~^~~~