Python Program to Print Middle most Node of a Linked List

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This is a Python program to print the middle node(s) of a linked list.

Problem Description

The program creates a linked list using data items input from the user and prints the middle node(s) of the linked list.

Problem Solution

1. Create a class Node with instance variables data and next.
2. Create a class LinkedList with instance variables head and last_node.
3. The variable head points to the first element in the linked list while last_node points to the last.
4. Define methods append and display inside the class LinkedList to append data and display the linked list respectively.
5. Define a function print_middle that prints the middle element(s) of the list.
6. The function print_middle iterates through the list to find its length and then iterates again to half its length. If the length of the list is even, it prints two middle elements.
7. Create an instance of LinkedList, append data to it and print its middle element(s).

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Program/Source Code

Here is the source code of a Python program to print the middle element(s) of a linked list.

class Node:
    def __init__(self, data):
       self.data = data
       self.next = None
 
 
class LinkedList:
    def __init__(self):
        self.head = None
        self.last_node = None
 
    def append(self, data):
        if self.last_node is None:
            self.head = Node(data)
            self.last_node = self.head
        else:
            self.last_node.next = Node(data)
            self.last_node = self.last_node.next
 
 
def print_middle(llist):
    current = llist.head
    length = 0
    while current:
        current = current.next
        length = length + 1
 
    current = llist.head
    for i in range((length - 1)//2):
        current = current.next
 
    if current:
        if length % 2 == 0:
            print('The two middle elements are {} and {}.'
                .format(current.data, current.next.data))
        else:
            print('The middle element is {}.'.format(current.data))
    else:
        print('The list is empty.')
 
 
a_llist = LinkedList()
 
data_list = input('Please enter the elements in the linked list: ').split()
for data in data_list:
    a_llist.append(int(data))
 
print_middle(a_llist)
Program Explanation

1. An instances of LinkedList is created.
2. The user is prompted to enter the data items for the list.
3. The function print_middle is called to print the middle element(s) of the list.

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Runtime Test Cases
Case 1:
Please enter the elements in the linked list: 1 2 3 4 5 6 7 8
The two middle elements are 4 and 5.
 
Case 2:
Please enter the elements in the linked list: 5
The middle element is 5.
 
Case 3:
Please enter the elements in the linked list: 3 1 0 4 2
The middle element is 0.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn | Youtube | Instagram | Facebook | Twitter