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Linked List Data Structure

Link list is a linear data Structure which can be defined as collection of objects called nodes that are randomly stored in the memory. A node contains two types of metadata i.e. data stored at that particular address and the pointer which contains the address of the next node in the memory. The last node of the list contains pointer to the null.

Why use linked list over array?

From the beginning, we are using array data structure to organize the group of elements that are stored individually in the memory. However, there are some advantage and disadvantage of array which should be known to decide which data structure will used throughout the program.

limitations

The size of array must be known in advance before using it in the program.
Increasing size of the array is a time taking process. It is almost impossible to expand the size of the array at run time.
All the elements in the array need to be contiguously stored in the memory. Inserting any element in the array needs shifting of all its predecessors.

So we introduce a new data structure to overcome these limitations.

Linked list is used because,

It allocates the memory dynamically. All the nodes of linked list are non-contiguously stored in the memory and linked together with the help of pointers.
Sizing is no longer a problem since we do not need to define its size at the time of declaration. List grows as per the program's demand and limited to the available memory space.

Let's code something

The smallest Unit: Node

class Node: def init(self, data): self.data = data # Assigns the given data to the node self.next = None # Initialize the next attribute to null

Now, we will see the types of linked list.

There are mainly four types of linked list,

Singly Link list
Doubly link list
Circular link list
Doubly circular link list

1. Singly linked list.

Simply think it is a chain of nodes in which each node remember(contains) the addresses of it next node.

Creating a linked list class

class LinkedList: def init(self): self.head = None # Initialize head as None

Inserting a new node at the beginning of a linked list

def insertAtBeginning(self, new_data):
    new_node = Node(new_data)  # Create a new node 
    new_node.next = self.head  # Next for new node becomes the   current head
    self.head = new_node  # Head now points to the new node

Inserting a new node at the end of a linked list

def insertAtEnd(self, new_data):
    new_node = Node(new_data)  # Create a new node
    if self.head is None:
        self.head = new_node  # If the list is empty, make the new node the head
        return
    last = self.head 
    while last.next:  # Otherwise, traverse the list to find the last node
        last = last.next
    last.next = new_node  # Make the new node the next node of the last node

Inserting a new node at the middle of a linked list

def insertAtPosition(self, data, position):
    new_node = Node(data)
    if position <= 0: #check if position is valid or not
        print("Position should be greater than 0")
        return
    if position == 1:
        new_node.next = self.head
        self.head = new_node
        return
    current_node = self.head
    current_position = 1
    while current_node and current_position < position - 1:   #Iterating to behind of the postion.
        current_node = current_node.next
        current_position += 1
    if not current_node:            #Check if Position is out of bound or not 
        print("Position is out of bounds")
        return
    new_node.next = current_node.next  #connect the intermediate node
    current_node.next = new_node

Printing the Linked list

def printList(self):
    temp = self.head # Start from the head of the list
    while temp:
        print(temp.data,end=' ') # Print the data in the current node
        temp = temp.next # Move to the next node
    print()  # Ensures the output is followed by a new line

Lets complete the code and create a linked list.

Connect all the code.

if __name__ == '__main__':
    llist = LinkedList()
    
    # Insert words at the beginning
    llist.insertAtBeginning(4) # <4>
    llist.insertAtBeginning(3) # <3> 4
    llist.insertAtBeginning(2) # <2> 3 4
    llist.insertAtBeginning(1) # <1> 2 3 4

    # Insert a word at the end
    llist.insertAtEnd(10)  # 1 2 3 4 <10>
    llist.insertAtEnd(7)   # 1 2 3 4 10 <7>

    #Insert at a random position 
    llist.insertAtPosition(9,4)  ## 1 2 3 <9> 4 10 7
    # Print the list
    llist.printList()

output:

1 2 3 9 4 10 7

Deleting a node from the beginning of a linked list

check the list is empty otherwise shift the head to next node.

def deleteFromBeginning(self): if self.head is None: return "The list is empty" # If the list is empty, return this string self.head = self.head.next # Otherwise, remove the head by making the next node the new head

Deleting a node from the end of a linked list

def deleteFromEnd(self): if self.head is None: return "The list is empty" if self.head.next is None: self.head = None # If there's only one node, remove the head by making it None return temp = self.head while temp.next.next: # Otherwise, go to the second-last node temp = temp.next temp.next = None # Remove the last node by setting the next pointer of the second-last node to None

Search in a linked list

def search(self, value): current = self.head # Start with the head of the list position = 0 # Counter to keep track of the position while current: # Traverse the list if current.data == value: # Compare the list's data to the search value return f"Value '{value}' found at position {position}" # Print the value if a match is found current = current.next position += 1 return f"Value '{value}' not found in the list"

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