Data Structures, Algorithms, & Applications in C++
Chapter 5, Exercise 37

The code is given below and in the file vectorListWithHalf.h. Test code is included in vectorListWithHalf.cpp. 
template<class T>
class vectorListWithHalf : public vectorList<T> 
{
   public:
      // constructor and destructor
      vectorListWithHalf(int initialCapacity = 10)
           : vectorList<T> (initialCapacity) {}

      void half();
};

template<class T>
void vectorListWithHalf<T>::half()
{// Remove all odd indexed elements.

   // move even indexed elements to new spots
   int listSize = (int) element->size();
   for (int i = 2; i < listSize; i += 2)
      (*element)[i/2] = (*element)[i];

   // destroy uneeded elements
   int newSize = (listSize + 1) / 2;
   element->erase(element->begin() + newSize, element->end());
}
The for loop is iterated O(listSize) times. Each iteration takes O(1) time. The complexity of the call to the vector method erase is O(listSize). Hence, the complexity of the function is O(listSize).