## BJP3 Exercise 16.18: doubleList

Author: Whitaker Brand (on 2013/04/01)

Write a method `doubleList` that doubles the size of a list by appending a copy of the original sequence to the end of the list. For example, if a variable `list` stores this sequence of values:

```[1, 3, 2, 7]
```

And we make the call of `list.doubleList();` then it should store the following values after the call:

```[1, 3, 2, 7, 1, 3, 2, 7]
```

Notice that it has been doubled in size by having the original sequence appear two times in a row. You may not make assumptions about how many elements are in the list. You may not call any methods of the class to solve this problem. If the original list contains n nodes, then you should construct exactly n nodes to be added. You may not use any auxiliary data structures to solve this problem (no array, ArrayList, stack, queue, String, etc). Your method should run in O(n) time where n is the number of nodes in the list.

Assume that you are adding this method to the `LinkedIntList` class as defined below:

```public class LinkedIntList {
private ListNode front;   // null for an empty list
...
}
```