View Compilation Error

0_0_13069647_22496.cpp:10:1: error: 'hash_set' does not name a type
 hash_set<long long>st[2];
 ^
0_0_13069647_22496.cpp: In function 'int main()':
0_0_13069647_22496.cpp:33:9: error: 'st' was not declared in this scope
         st[0].clear();
         ^