Linked List

Instructions

Your team has decided to use a doubly linked list to represent each train route in the schedule. Each station along the train's route will be represented by a node in the linked list.

You don't need to worry about arrival and departure times at the stations. Each station will simply be represented by a number.

Routes can be extended, adding stations to the beginning or end of a route. They can also be shortened by removing stations from the beginning or the end of a route.

Sometimes a station gets closed down, and in that case the station needs to be removed from the route, even if it is not at the beginning or end of the route.

The size of a route is measured not by how far the train travels, but by how many stations it stops at.

How this Exercise is Structured in Python

While linked lists can be implemented in a variety of ways with a variety of underlying data structures, we ask here that you implement your linked list in an OOP fashion.

In the stub file, you will see the start of a Node class, as well as a LinkedList class. Your Node class should keep track of its value, as well as which other nodes preceed or follow. Your push, pop, shift, unshift, and the special method for len should be implemented in the LinkedList class. You might also find it useful to implement a special iter method for iteration.

Unlike the core exercise, we will be testing error conditions by calling pop and shift on empty LinkedLists, so you will need to raise errors appropriately.

Finally, we would like you to implement delete in addition to the methods outlined above. delete will take one argument, which is the value to be removed from the linked list. If the value appears more than once, only the first occurrence should be removed.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.

This particular exercise requires that you use the raise statement to "throw" a ValueError when a node value being delete()-ed is not found in the linked list. Additionally, an IndexError should be thrown if there are no nodes left to pop(). The tests will only pass if you both raise these exceptions and include messages with them.

To raise a ValueError with a message, write the message as an argument to the exception type:

# When the value passed to `delete()` is not found.
if not found:
    raise ValueError("Value not found")

To raise an IndexError with a message, write the message as an argument to the exception type:

# When pop() is called and there are no nodes left in the linked list
if self.length == 0:
    raise IndexError("List is empty")

Special Methods in Python

The tests for this exercise will also be calling len() on your LinkedList. In order for len() to work, you will need to create a __len__ special method. For details on implementing special or "dunder" methods in Python, see Python Docs: Basic Object Customization and Python Docs: object.len(self).

We also recommend creating a special __iter__ method to help with iterating over your linked list.