Write an array-based implementation of the ADT list that expands the size of the array of list entries as needed so that the list can always accommodate a new entry.

Also reduce the size of the array as needed to accommodate several removals. When the size of the array is greater than 20 and the number of entries in the list is less than half the size of the array, reduce the size of the array so that it is three quarters of its current size.

Note: In expanding the array - when an element is added to a full array, double the size of the array.