Python Program For Inserting Node In The Middle Of The Linked List

Given a linked list containing n nodes. The problem is to insert a new node with data x at the middle of the list. If n is even, then insert the new node after the (n/2)th node, else insert the new node after the (n+1)/2th node.

Examples:

Input : list: 1->2->4->5
        x = 3
Output : 1->2->3->4->5

Input : list: 5->10->4->32->16
        x = 41
Output : 5->10->4->41->32->16

Method 1(Using length of the linked list):
Find the number of nodes or length of the linked using one traversal. Let it be len. Calculate c = (len/2), if len is even, else c = (len+1)/2, if len is odd. Traverse again the first c nodes and insert the new node after the cth node.  

Python3

# Python3 implementation to insert node
# at the middle of a linked list

# Node class
class Node:

    # constructor to create a new node
    def __init__(self, data):
        self.data = data
        self.next = None

# function to insert node at the
# middle of linked list given the head
def insertAtMid(head, x):

    if(head == None): #if the list is empty
        head = Node(x)
    else:

        # create a new node for the value
        # to be inserted
        newNode = Node(x)

        ptr = head
        length = 0

        # calculate the length of the linked
        # list
        while(ptr != None):
            ptr = ptr.next
            length += 1

        # 'count' the number of node after which
        # the new node has to be inserted
        if(length % 2 == 0):
            count = length / 2
        else:
            (length + 1) / 2

        ptr = head

        # move ptr to the node after which
        # the new node has to inserted
        while(count > 1):
            count -= 1
            ptr = ptr.next

        # insert the 'newNode' and adjust
        # links accordingly
        newNode.next = ptr.next
        ptr.next = newNode

# function to display the linked list
def display(head):
    temp = head
    while(temp != None):
        print(str(temp.data), end = " ")
        temp = temp.next

# Driver Code

# Creating the linked list 1.2.4.5
head = Node(1)
head.next = Node(2)
head.next.next = Node(4)
head.next.next.next = Node(5)

print("Linked list before insertion: ", end = "")
display(head)

# inserting 3 in the middle of the linked list.
x = 3
insertAtMid(head, x)

print("
Linked list after insertion: " , end = "")
display(head)

# This code is contributed by Pranav Devarakonda

Output:

Linked list before insertion: 1 2 4 5
Linked list after insertion: 1 2 3 4 5

Time Complexity: O(n)

Space complexity: O(n) where n is the number of nodes in a linked list

Method 2(Using two pointers):
Based on the tortoise and hare algorithm which uses two pointers, one known as slow and the other known as fast. This algorithm helps in finding the middle node of the linked list. It is explained in the front and black split procedure of this post. Now, you can insert the new node after the middle node obtained from the above process. This approach requires only a single traversal of the list. 

Python3

# Python implementation to insert node
# at the middle of the linked list

# Node Class
class Node :
    def __init__(self, d):
        self.data = d
        self.next = None

class LinkedList:

    # function to insert node at the
    # middle of the linked list
    def __init__(self):
        self.head = None

    # 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

    def insertAtMid(self, x):

        # if list is empty
        if (self.head == None):
            self.head = Node(x)

        else:

            # get a new node
            newNode = Node(x)

            # assign values to the slow
            # and fast pointers
            slow = self.head
            fast = self.head.next

            while (fast != None and
                   fast.next != None):

                # move slow pointer to next node
                slow = slow.next

                # move fast pointer two nodes
                # at a time
                fast = fast.next.next

            # insert the 'newNode' and
            # adjust the required links
            newNode.next = slow.next
            slow.next = newNode

    # function to display the linked list
    def display(self):
        temp = self.head
        while (temp != None):
            print(temp.data, end = " "),
            temp = temp.next

# Driver Code

# Creating the list 1.2.4.5
ll = LinkedList()
ll.push(5)
ll.push(4)
ll.push(2)
ll.push(1)
print("Linked list before insertion: "),
ll.display()

x = 3
ll.insertAtMid(x)

print("
Linked list after insertion: "),
ll.display()

# This code is contributed by prerna saini

Output:

Linked list before insertion: 1 2 4 5
Linked list after insertion: 1 2 3 4 5

Time Complexity: O(n)

Space complexity: O(n) where n is size of linked list

Please refer complete article on Insert node into the middle of the linked list for more details!