Doubly linked list in Data Structure

The doubly linked list is a list in which each node has three parts. one info part and two links part. one link part refers to the net node and the other link nodes refer to the previous node. As you see in the given below image first node’s previous link part is always none and the last node’s next link is always none.

Doubly linked list Data structures and algorithms

Advantages

it can be traversed in both forward and backward directions. Implementation of some operations is easier, like insertion, and deletion can be done with only a single reference.

Disadvantages

Extra space is required for storing the previous link. While inserting and deleting some extra steps are required to maintain the previous link.

Doubly Linked List Program in Python programming

class Node(object):

    def __init__(self, value):
        self.info = value
        self.prev = None        self.next = None

class DoubleLinkedList(object):

    def __init__(self):
        self.start = None
    def display_list(self):
        if self.start is None:
            print("LIst is empty")
            return
        print("List is : ")
        p = self.start
        while p is not None:
            print(p.info, " ", end='')
            p = p.next
        print()

    def insert_in_beginning(self, data):
        temp = None(data)
        temp.next = self.start
        self.start.prev = temp
        self.start = temp

    def insert_in_empty_list(self, data):
        temp = Node(data)
        self.start = temp

    def insert_at_end(self, data):
        temp = Node(data)
        p = self.start
        while p.next is not None:
            p = p.next
        p.next = temp
        temp.prev = p

    def create_list(self):
        n = int(input("Enter the number of nodes : "))
        if n == 0:
            return        data = int(input("Enter the first element to be inserted : "))
        self.insert_in_empty_list(data)

        for i in range(n - 1):
            data = int(input("Enter the next element to be inserted : "))
            self.insert_at_end(data)

    def insert_after(self, data, x):
        temp = Node(data)
        p = self.start
        while p is not None:
            if p.info == x:
                break            p = p.next

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

    def insert_before(self, data, x):
        if self.start is None:
            print("List is empty")
            return
        if self.start.info == x:
            temp = Node(data)
            temp.next = self.start
            self.start.prev = temp
            self.start = temp
            return
        p = self.start
        while p is not None:
            if p.info == x:
                break            p = p.next

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

    def delete_first_node(self):
        if self.start is None:
            return        if self.start.next is None:
            self.start = None            return        self.start = self.start.next
        self.start.prev = None
    def delete_last_node(self):
        if self.start is None:
            return        if self.start.next is None:
            self.start = None            return
        p = self.start
        while p.next != None:
            p = p.next
        p.prev.next = None
    def delete_node(self, x):
        if self.start is None:
            return        if self.start.next is None:
            if self.start.info == x:
                self.start = None            else:
                print(x, " not found")
            return
        if self.start.info == x:
            self.start = self.start.next
            self.start.prev = None            return
        p = self.start.next
        while p.next is not None:
            if p.info == x:
                break
            p = p.next

        if p.next is not None:
            p.prev.next = p.next
            p.next.prev = p.prev
        else:
            if p.info == x:
                p.prev.next
            else:
                print(x, " not found")

    def reverse_list(self):
        if self.start is None:
            return
        p1 = self.start
        p2 = p1.next
        p1.next = None        p1.prev = p2
        while p2 is not None:
            p2.prev = p2.next
            p2.next = p1
            p1 = p2
            p2 = p2.prev
        self.start = p1


#################################################################3
list = DoubleLinkedList()

list.create_list()

while True:
    print("1. Display list")
    print("2. Insert in empty list")
    print("3. Insert a node in the beginning of the list")
    print("4. Insert a node at the end of the list")
    print("5. Insert a node after a specified node")
    print("6. Insert a node before a specified node")
    print("7. Delete first node")
    print("8. Delete last node")
    print("9. Delete any node")
    print("10. Reverse the list")
    print("11. 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_in_empty_list(data)
    elif option == 3:
        data = int(input("Enter the element to be inserted : "))
        list.insert_in_beginning(data)
    elif option == 4:
        data = int(input("Enter the element to be inserted : "))
        list.insert_at_end(data)
    elif option == 5:
        data = int(input("Enter the element to be inserted : "))
        x = int(input("Enter the element after which to insert : "))
        list.insert_after(data, x)
    elif option == 6:
        data = int(input("Enter the element to be inserted : "))
        x = int(input("Enter the element before which to inserted : "))
        list.insert_before(data, x)
    elif option == 7:
        list.delete_first_node()
    elif option == 8:
        list.delete_last_node()
    elif option == 9:
        data = int(input("Enter the element to be deleted : "))
        list.delete_node(data)
    elif option == 10:
        list.reverse_list()
    elif option == 11:
        break    else:
        print("Wrong option")
    print()