View Compilation Error

0_0_18878792_5944.cpp: In function 'int main()':
0_0_18878792_5944.cpp:10:2: error: 'vector' was not declared in this scope
  vector<int> a;
  ^
0_0_18878792_5944.cpp:10:9: error: expected primary-expression before 'int'
  vector<int> a;
         ^
0_0_18878792_5944.cpp:13:3: error: 'a' was not declared in this scope
   a.clear();
   ^