How to reverse a linked list?
You probably heard this question a billion times already, may it be from coding interviews, colleagues, and even some YouTube influencers saying you aint a programmer if you don't know how to reverse a linked list (talking to you TechLead).
No worries, there's no shame in not knowing that.
However, it is my duty as your data structures teacher, to teach you that and pass on knowledge from one programmer to another.
So without further ado, let's master the linked list.
- Demystifying Linked Lists
- Where the hell are they used?
- Types of Linked Lists
- Implementations of Linked Lists
- How to reverse a god damn linked list?
- Additional Resources
When I first started studying data structures, I was pretty terrified of linked lists.
I don't know what it is, but it sounds so complicated.
But boy was I wrong.
It's actually far simpler than I thought.
So to begin with, let us define the linked list.
- A linked list is a data structure.
- It's linear, meaning elements are in a sequence (ex. 1 2 3 4 5).
- Elements of a linked list are NOT stored sequentially to each other in memory(RAM).
- Elements are linked to each other's using pointers.
Below is an image visualizing linked lists:
Let us break this image down:
- Head: The first element of a linked list (in this case it is the element at memory address 3200)
- Content: Your data (can be anything you'd want ex. string, ints, floats, etc...)
- Pointer: Address in memory to the next element (node).
- Tail: Last element of a linked list (you can identify a tail by it having always a pointer of next element to null)
As programmers, our first instinct is to see how we can use this in our current or new projects, and this is valid thinking.
We don't wanna be studying something that we might never use.
Well, that is why I've compiled down a list of use-cases for linked lists.
- Implementation of Stacks and Queues (FIFO)
- Implementation of Graphs
- Dynamic memory allocation
- Maintaining directory of names
- Performing arithmetic operations on long integers
- Manipulation of Polynomials
- Representing sparse matrices
There are three types of linked lists:
A singly linked list is a type of linked list that is unidirectional, that is, it can be traversed in only one direction from head to the last node(tail).
A doubly linked list is a type of linked list that is bidirectional, that is, it can be traversed in two directions, from head to the last node(tail) and vice-versa.
A circular linked list is a type of linked list, in which the head points to the tail, and the tail points to the head. Both Singly Linked List and Doubly Linked List can be made into a circular linked list.
Keep in mind when measuring the complexity of a data structure, we always look at the worst-case scenario. So we can imagine that the size of this linked list is in the millions or even billions.
Both single and doubly linked list has to traverse the whole linked list to find the specific element that we are searching for. That is why the complexity for is O(n).
Search is similar to access time, so it is also O(n).
Linked lists are used best in insertion and deletion due to the elements not needing to be explicitly sequential in memory.
Insertion is pretty simple.
For example, if you want to add an element to the beginning of a linked list(head). You simply create the element in some memory location and you point it to the previous head. This is why the time complexity is O(1).
Similar to insertion, the time complexity is O(1), because to delete your not gonna have to go through the whole list, shifting elements. You simply manipulate the pointers of the previous and next element that you want to delete.
Usually, to represent a node in a linked list, we create a class with these properties:
class Node: def __init__(self, data=None): self.data = data self.nextval = None
- Data: value of the data (string, int, etc...).
- Nextval: pointer or reference to the next element in the list.
The standard linked list interface requires us to implement these methods:
list- Display all elements/nodes.
push- insert at the end of the list.
insertAtBeginning- insert at the beginning of the list
insert- inserting in between two Data Nodes
remove- remove an existing node using the key for that node
Our base class will look like this:
class SLinkedList: # Single Linked List def __init__(self): self.headval = None
Let's implement the methods one by one.
class SLinkedList: def __init__(self): self.headval = None def list(self): printval = self.headval while printval is not None: print (printval.dataval) printval = printval.nextval
class SLinkedList: def __init__(self): self.headval = None def insertAtBeginning(self,newdata): NewNode = Node(newdata) # Update the new nodes next val to existing node NewNode.nextval = self.headval self.headval = NewNode
class SLinkedList: def __init__(self): self.headval = None # Function to add newnode def push(self, newdata): NewNode = Node(newdata) if self.headval is None: self.headval = NewNode return laste = self.headval while(laste.nextval): laste = laste.nextval laste.nextval=NewNode
class SLinkedList: def __init__(self): self.headval = None def insertBetween(self,middle_node,newdata): if middle_node is None: print("The mentioned node is absent") return NewNode = Node(newdata) NewNode.nextval = middle_node.nextval middle_node.nextval = NewNode
class SLinkedList: def __init__(self): self.head = None def remove(self, Removekey): HeadVal = self.head if (HeadVal is not None): if (HeadVal.data == Removekey): self.head = HeadVal.next HeadVal = None return while (HeadVal is not None): if HeadVal.data == Removekey: break prev = HeadVal HeadVal = HeadVal.next if (HeadVal == None): return prev.next = HeadVal.next HeadVal = None
Keep in mind, here we are implementing a single-linked list. Implementing a double-linked list is similar, apart from the extra
prev variable which will store the pointer or reference to the previous element. It will look something like this.
class Node: def __init__(self, data=None): self.data = data self.next = None self.prev = None
Congratulations, you know about linked lists.
Now, coming back to our main question.
How do we reverse a linked list?
Think about it, and then proceed to read the answer below.
To help you imagine this:
Input: 1→ 2→ 3→ 4
Output: 4 → 3 → 2 → 1
There are multiple ways to reverse a linked list, which includes:
- Iterative Method
- Recursive Method
- Stack Method
For the sake of simplicity, we are going to use the iterative method.
- Initialize three pointers
curras head and
- Iterate through the linked list. In the loop, do the following:
// Before changing next of current, // store next node next = curr->next // Now change next of current // This is where actual reversing happens curr->next = prev // Move prev and curr one step forward prev = curr curr = next
# Python program to reverse a linked list # Time Complexity: O(n) # Space Complexity: O(1) # Node class class Node: # Constructor to initialize the node object def __init__(self, data): self.data = data self.next = None class LinkedList: # Function to initialize head def __init__(self): self.head = None # Function to reverse the linked list def reverse(self): prev = None current = self.head while(current is not None): next = current.next current.next = prev prev = current current = next self.head = prev # Function to insert a new node at the beginning def push(self, new_data): new_node = Node(new_data) new_node.next = self.head self.head = new_node # Utility function to print the linked LinkedList def printList(self): temp = self.head while(temp): print temp.data, temp = temp.next # Driver code llist = LinkedList() llist.push(20) llist.push(4) llist.push(15) llist.push(85) print "Given Linked List" llist.printList() llist.reverse() print "\nReversed Linked List" llist.printList() # This code is contributed by Nikhil Kumar Singh(nickzuck_007)
In summary today we learned:
- Linked Lists and their properties.
- Where it is used
- How to reverse one
I hope you learned something today, and if you got any questions/suggestions feel free to comment them down in the description.