It’s important to know about data types and one that comes up fairly regularly is that of Linked Lists.
Let’s write the following base code to create a Linked List.
# Defintion of a single Node class Node: # takes input data and next node def __init__(self, data = None, next=None): self.data = data self.next = next # Definition of a Linked List class LinkedList: def __init__(self): self.head = None # Insert a node into our Linked List def insert(self, data): newNode = Node(data) if(self.head): current = self.head while(current.next): current = current.next current.next = newNode else: self.head = newNode # Print our Linked List so we can see how it looks def render(self): current = self.head while(current): print(current.data) current = current.next
Now that we have a base class, let’s insert a couple of nodes and print it out to see what we are working with:
# Define our LinkedList ll = LinkedList() # Insert a couple of nodes ll.insert(1) ll.insert(2) ll.insert(3) ll.insert(4) # Render it out to the screen ll.render()
What do we need to do to reverse this linked list?
Let’s start by creating a
We should know that:
- All the links start pointing in the opposite direction
- The head starts to point to the last element of the list
Using an iterative approach, let’s start writing our function.
We will need to create a couple of variables to keep track of the position of each of our nodes as we loop through.
Some things to note:
previous initially points to None. This becomes the
head on consecutive calls.
current points to the first element. This is the current
head element point.
following points to the second element. This is the
next element point.
# takes a `list` input def reverse(list): # initialize our 3 main variables previous = None current = list.head following = current.next # keep looping until at the end of the list while current: # reverse the link current.next = previous previous = current current = following # if there are more nodes and this isn't the end if following: following = following.next # set the head to the previous item list.head = previous
Let’s test it!
# Define a LinkedList ll = LinkedList() # Insert some nodes ll.insert(1) ll.insert(2) ll.insert(3) ll.insert(4) # Render our list in it's current order print("Linked List") ll.render() # Reverse the list reverse(ll) # Render our list in it's new order print("Reversed Linked List") ll.render()
You know know how to reverse a list in Python!