The header linked list is a list that has a header node at the beginning of the linked list. and the head always refers to the header node of the list.

What is Header Node

the info part of the header node is always None. therefor we can use this part to store some useful information about the linked list. like storing the number of nodes in a list and some of the value of linked list etc.

and the linked part of the header node is always referred to as the first node of the linked list.



Advantages of Header linked list

If the list is empty then always the header node is available in the linked list. so we don't need to check whether the linked list is empty or not.


we can easily perform operations like insertion and deletion because we don't need to check if the list is empty or not.

Python program for Implementation of Header linked list

class Node(object):

    def __init__(self,value):
        self.info = value
        self.link = None
class HeaderLinkedList(object):

    def __init__(self):
        self.head = Node(0)

    def display_list(self):
        if self.head.link == None:
            print("List is empty")
            return
        p =self.head.link
        print("List is : ")
        while p is not None:
            print(p.info, " ",end='')
            p = p.link
        print()

    def create_list(self):
        n = int(input("Enter the number of nodes : "))
        for i in range(n):
            data = int(input("Enter the element to be inserted : "))
            self.insert_at_end(data)

    def insert_at_end(self,data):
        temp = Node(data)

        p = self.head
        while p.link is not None:
            p = p.link
        p.link = temp

    def insert_before(self,data,x):
        p = self.head
        while p.link is not None:
            if p.link.info == x:
                break            p = p.link

        if p.link is None:
            print(x, " not present in the list")
        else:
            temp = Node(data)
            temp.link = p.link
            p.link = temp

    def insert_at_position(self,data,x):
        p = self.head
        i = 1        while i <= k-1 and p is not None:
            p = p.link
            i += 1
        if p is None:
            print("You can insert only upto ", (i-1), " th position")
        else:
            temp = Node(data)
            temp.link = p.link
            p.link  = temp

    def delete_node(self,data):
        p = self.head
        while p.link is not None:
            if p.link.info == data:
                break            p = p.link

        if p.link == None:
            print(data, " not found")
        else:
            p.link = p.link.link

    def reverse_list(self):
        prev = None        p = self.head.link
        while p is not None:
            next = p.link
            p.link = prev
            prev = p
            p = next

        self.head.link = prev

list = HeaderLinkedList()

list.create_list()

while True:
    print("1. Display list")
    print("2. Insert a node at the end of the list")
    print("3. Insert a node before a specified node")
    print("4. Insert a node at a given position")
    print("5. Delete a node")
    print("6. Reverse the list")
    print("9. Quit")

    option = int(input("Enter your choice : "))

    if option == 1:
        list.display_list()
    elif option == 2:
        data = int(input("Enter the element to be inserted : "))
        list.insert_at_end(data)
    elif option == 3:
        data = int(input("Enter the element to be inserted : "))
        x = int(input("Enter the element before which to insert : "))
        list.insert_before(data,x)
    elif option == 4:
        data = int(input("Enter the element to be inserted : "))
        k = int(input("Enter the position at which to insert : "))
        list.insert_at_position(data,x)
    elif option == 5:
        data = int(input("Enter the element to be deleted : "))
        list.delete_node(data)
    elif option == 7:
        break    else:
        print("Wrong option")
    print()

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