MiddleNode.py

# Python program to find middle node of a singly linked list


# Node class
class Node:

    # Constructor to initialise data and next
    def __init__(self, data=None):
        self.data = data
        self.next = None


class SinglyLinkedList:

    # Constructor to initialise head
    def __init__(self):
        self.head = None

    # Function to find middle node of a linked list
    def find_mid(self):
        fast = self.head
        slow = self.head

        # Make fast run twice the speed of slow
        # when fast is at the last of the list
        # slow will be at the mid node
        while fast is not None:
            fast = fast.next
            if fast is not None:
                fast = fast.next
                slow = slow.next

        return slow

    # Function to Insert data at the beginning of the linked list
    def insert_at_beg(self, data):
        node = Node(data)
        node.next = self.head
        self.head = node

    # Function to print the linked list
    def print_data(self):
        current = self.head
        while current is not None:
            print(current.data, '-> ', end='')
            current = current.next
        print('None')

if __name__ == '__main__':
    linked_list = SinglyLinkedList()
    linked_list.insert_at_beg(9)
    linked_list.insert_at_beg(8)
    linked_list.insert_at_beg(7)
    linked_list.insert_at_beg(6)
    linked_list.insert_at_beg(5)
    linked_list.insert_at_beg(4)
    linked_list.insert_at_beg(3)
    linked_list.insert_at_beg(2)
    linked_list.insert_at_beg(1)
    linked_list.print_data()
    # call the find_mid function
    mid = linked_list.find_mid()
    # print the middle node if not None
    if mid is not None:
        print(mid.data)