An Introduction to Data Structures

Implementing Linked Lists (Implementing Iterators)

Note that it is illegal to advance the iterator once it is in the past-the-end position.
```
void Iterator::next()
{
   assert(position != NULL)
   position = position->next;
}
```
The previous function is similar.

When the iterator points to the first element in the list, it is illegal to move it further backward.

void Iterator::previous()
{
   if (position == NULL)
      position = last;
   else
      position = position->previous;
   assert(position != NULL);
}

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