View Compilation Error

0_0_28177388_9061.cpp: In function 'int main()':
0_0_28177388_9061.cpp:30:3: error: 'hash_set' was not declared in this scope
   hash_set<int> s;
   ^
0_0_28177388_9061.cpp:30:3: note: suggested alternative:
In file included from 0_0_28177388_9061.cpp:2:0:
GCC4.9.2/x86_64-w64-mingw32/include/c++/backward/hash_set:84:11: note:   '__gnu_cxx::hash_set'
     class hash_set
           ^
0_0_28177388_9061.cpp:30:12: error: expected primary-expression before 'int'
   hash_set<int> s;
            ^
0_0_28177388_9061.cpp:33:4: error: 's' was not declared in this scope
    s.insert(p[i]);
    ^
0_0_28177388_9061.cpp:36:18: error: 's' was not declared in this scope
   printf("%d\n", s.size() - 1);
                  ^