Write a method collapse that collapses a list
of integers by replacing each successive pair of integers with the sum of
the pair. For example, if a variable called list stores this sequence:

[7, 2, 8, 9, 4, 13, 7, 1, 9, 10]

and the following call is made:

list.collapse();

The first pair should be collapsed into 9 (7 + 2), the second pair should be
collapsed into 17 (8 + 9), the third pair should be collapsed into 17 (4 +
13) and so on to yield:

[9, 17, 17, 8, 19]

If the list stores an odd number of elements, the final element is not
collapsed. For example, the sequence:

[1, 2, 3, 4, 5]

would collapse into:

[3, 7, 5]

You are writing a method for the ArrayIntList class discussed in lecture:

public class ArrayIntList {
private int[] elementData; // list of integers
private int size; // current # of elements in the list
<methods>
}

You are not to call any other ArrayIntList methods to solve this problem,
you are not allowed to define any auxiliary data structures (no array,
ArrayList, etc), and your solution must run in O(n) time where n is the
length of the list to be collapsed.