Python program to insert a new node at the middle of the Doubly Linked List.

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Python program to insert a new node at the middle of the Doubly Linked List.

In this program, we create a doubly linked list and insert a new node in the middle of the list. If the list is empty, both head and tail will point to the new node. If the list is not empty, then we will calculate the size of the list and divide it by 2 to get the mid-point of the list where a new node needs to be inserted.

Python program to insert a new node at the middle of the Doubly Linked List

Consider the above diagram; a new node needs to be added to the middle of the list. First, we calculate the size which in this case is 4. So, to get the mid-point, we divide it by 2 and store it in a variable mid. Node current will point to head. First, we iterate through the list until current points to the mid position. Define another node temp which point to node next to current. Insert the new node between current and temp

ALGORITHM:

  1. Define a Node class which represents a node in the list. It will have three properties: data, previous which will point to the previous node and next which will point to the next node.
  2. Define another class for creating the doubly linked list, and it has two nodes: head and tail. Initially, head and tail will point to null.
  3. addNode() will add node to the list:
  • It first checks whether the head is null, then it will insert the node as the head.
  • Both head and tail will point to a newly added node.
  • Head’s previous pointer will point to null and tail’s next pointer will point to null.
  • If the head is not null, the new node will be inserted at the end of the list such that new node’s previous pointer will point to tail.
  • The new node will become the new tail. Tail’s next pointer will point to null.

a. addInMid() will add a node to the middle of the list:

  • It first checks whether the head is null (empty list), then it will insert the node as the head.
  • Both head and tail will point to a newly added node.
  • If the list is not empty, then we calculate size and divide it by 2 to get the mid-point.
  • Define node current that will point to head and iterate through the list till current will point to the mid node.
  • Define another node temp which will point to node next to current.
  • The new node will be inserted after current and before temp such that current will point to the new node and the new node will point to temp.

a. display() will show all the nodes present in the list.

  • Define a new node ‘current’ that will point to the head.
  • Print current.data till current points to null.
  • Current will point to the next node in the list in each iteration.

PROGRAM:

  1. #Represent a node of doubly linked list  
  2. class Node:
  3.     def __init__(self,data):
  4.         self.data = data;
  5.         self.previous = None;
  6.         self.next = None;
  8. class InsertMid:
  9.     #Represent the head and tail of the doubly linked list  
  10.     def __init__(self):
  11.         self.head = None;
  12.         self.tail = None;
  13.         self.size = 0;
  15.     #addNode() will add a node to the list  
  16.     def addNode(self, data):
  17.         #Create a new node  
  18.         newNode = Node(data);
  20.         #If list is empty  
  21.         if(self.head == None):
  22.             #Both head and tail will point to newNode  
  23.             self.head = self.tail = newNode;
  24.             #head’s previous will point to None  
  25.             self.head.previous = None;
  26.             #tail’s next will point to None, as it is the last node of the list  
  27.             self.tail.next = None;
  28.         else:
  29.             #newNode will be added after tail such that tail’s next will point to newNode  
  30.             self.tail.next = newNode;
  31.             #newNode’s previous will point to tail  
  32.             newNode.previous = self.tail;
  33.             #newNode will become new tail  
  34.             self.tail = newNode;
  35.             #As it is last node, tail’s next will point to None  
  36.             self.tail.next = None;
  37.         #Size will count the number of nodes present in the list  
  38.         self.size = self.size + 1;
  40.     #addInMid() will add a node to the middle of the list  
  41.     def addInMid(self, data):
  42.         #Create a new node  
  43.         newNode = Node(data);
  45.         #If list is empty  
  46.         if(self.head == None):
  47.             #Both head and tail will point to newNode  
  48.             self.head = self.tail = newNode;
  49.             #head’s previous will point to None  
  50.             self.head.previous = None;
  51.             #tail’s next point to None, as it is the last node of the list  
  52.             self.tail.next = None;
  53.         else:
  54.             #current will point to head  
  55.             current = self.head;
  57.             #Store the mid position of the list  
  58.             mid = (self.size//2) if(self.size % 2 == 0)  else ((self.size+1)//2);
  60.             #Iterate through list till current points to mid position  
  61.             for i in range(1, mid):
  62.                 current = current.next;
  64.             #Node temp will point to node next to current  
  65.             temp = current.next;
  66.             temp.previous = current;
  68.             #newNode will be added between current and temp  
  69.             current.next = newNode;
  70.             newNode.previous = current;
  71.             newNode.next = temp;
  72.             temp.previous = newNode;
  73.         self.size = self.size + 1;
  75.     #display() will print out the nodes of the list  
  76.     def display(self):
  77.         #Node current will point to head  
  78.         current = self.head;
  79.         if(self.head == None):
  80.             print(“List is empty”);
  81.             return;
  82.         while(current != None):
  83.             #Prints each node by incrementing pointer.  
  84.             print(current.data),
  85.             current = current.next;
  87.         print();
  89. dList = InsertMid();
  90. #Add nodes to the list  
  91. dList.addNode(1);
  92. dList.addNode(2);
  94. print(“Original list: “);
  95. dList.display();
  97. #Adding node ‘3’ in the middle  
  98. dList.addInMid(3);
  99. print( “Updated List: “);
  100. dList.display();
  102. #Adding node ‘4’ in the middle  
  103. dList.addInMid(4);
  104. print(“Updated List: “);
  105. dList.display();
  107. #Adding node ‘5’ in the middle  
  108. dList.addInMid(5);
  109. print(“Updated List: “);
  110. dList.display();

Output:

Original list: 
1 2 
Updated List: 
1 3 2 
Updated List: 
1 3 4 2 
Updated List: 
1 3 5 4 2
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