Write a method isConsecutive that returns
true if a list of integers contains a sequence of consecutive integers and
that returns false otherwise. Consecutive integers are integers that come
one after the other, as in 5, 6, 7, 8, 9, etc. For example, if a variable
called list stores these values:

[3, 4, 5, 6, 7, 8, 9, 10]

then the call:

list.isConsecutive()

should return true. If the list had instead contained this sequence:

[3, 4, 5, 6, 7, 8, 9, 10, 12]

then the call should return false because the numbers 10 and 12 are not
consecutive. The following sequence of values would be consecutive except
for the fact that it appears in reverse order, so the method would return
false in this case:

[3, 2, 1]

Any list with fewer than two values should be considered to be consecutive.

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