/in/foo.cc:16:48: error: ISO C++ forbids declaration of 'operator<' with no type [-fpermissive]
  friend operator < (const Node &a,const Node &b){
                                                ^
/in/foo.cc: In function 'int cantor(const char*)':
/in/foo.cc:57:10: error: reference to 'hash' is ambiguous
   cnt += hash[i] * tmp;
          ^~~~
/in/foo.cc:11:5: note: candidates are: int hash [10]
 int hash[10];
     ^~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/functional_hash.h:58:12: note:                 template<class _Tp> struct std::hash
     struct hash;
            ^~~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:139:2: error: reference to 'hash' is ambiguous
  hash[0]=1;
  ^~~~
/in/foo.cc:11:5: note: candidates are: int hash [10]
 int hash[10];
     ^~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/functional_hash.h:58:12: note:                 template<class _Tp> struct std::hash
     struct hash;
            ^~~~
/in/foo.cc:141:3: error: reference to 'hash' is ambiguous
   hash[i] = hash[i-1] * i;
   ^~~~
/in/foo.cc:11:5: note: candidates are: int hash [10]
 int hash[10];
     ^~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/functional_hash.h:58:12: note:                 template<class _Tp> struct std::hash
     struct hash;
            ^~~~
/in/foo.cc:141:13: error: reference to 'hash' is ambiguous
   hash[i] = hash[i-1] * i;
             ^~~~
/in/foo.cc:11:5: note: candidates are: int hash [10]
 int hash[10];
     ^~~~
In file included from /usr/include/c++/6/bits/basic_string.h:5643:0,
                 from /usr/include/c++/6/string:52,
                 from /usr/include/c++/6/bits/locale_classes.h:40,
                 from /usr/include/c++/6/bits/ios_base.h:41,
                 from /usr/include/c++/6/ios:42,
                 from /usr/include/c++/6/ostream:38,
                 from /usr/include/c++/6/iostream:39,
                 from /in/foo.cc:1:
/usr/include/c++/6/bits/functional_hash.h:58:12: note:                 template<class _Tp> struct std::hash
     struct hash;
            ^~~~
/in/foo.cc: In function 'int search(Node)':
/in/foo.cc:135:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^