/in/foo.cc: In function 'int main()':
/in/foo.cc:24:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for(int i=s.find(s1);i!=string::npos;i=s.find(s1,i+len2))num++;
                               ^
/in/foo.cc:23:19: warning: unused variable 'len1' [-Wunused-variable]
         int num=0,len1=s.length(),len2=s1.length();
                   ^