View Compilation Error

0_0_18095620_1084.cpp:46:1: error: 'hash_map' does not name a type
 hash_map<int, int> G;
 ^
0_0_18095620_1084.cpp: In function 'void DFS(int, int)':
0_0_18095620_1084.cpp:52:4: error: 'G' was not declared in this scope
    G[h]= C;
    ^
0_0_18095620_1084.cpp: In function 'int main()':
0_0_18095620_1084.cpp:88:4: error: 'G' was not declared in this scope
    G.clear();
    ^