use static allocator instead

vector/src/vector.hpp

@@ -16,7 +16,7 @@ namespace sjtu {
  */
 template <typename T>
 class vector {
-  std::allocator<T> alloc;
+  static std::allocator<T> alloc;
   size_t allocated_length;
   size_t current_length;
   T *raw_beg, *raw_end;
@@ -504,6 +504,8 @@ class vector {
     }
   }
 };
+template<typename T>
+std::allocator<T> vector<T>::alloc;
 
 }  // namespace sjtu