Python Program For Making Middle Node Head In A Linked List

Given a singly linked list, find middle of the linked list and set middle node of the linked list at beginning of the linked list. 
Examples:

Input: 1 2 3 4 5 
Output: 3 1 2 4 5

Input: 1 2 3 4 5 6
Output: 4 1 2 3 5 6 

The idea is to first find middle of a linked list using two pointers, first one moves one at a time and second one moves two at a time. When second pointer reaches end, first reaches middle. We also keep track of previous of first pointer so that we can remove middle node from its current position and can make it head. 

Python3

# Python3 program to make middle node
# as head of Linked list

# Linked List node
class Node:
    def __init__(self, data):
        self.data = data
        self.next = None

# function to get the middle node
# set it as the beginning of the
# linked list
def setMiddleHead(head):
    if(head == None):
        return None

    # To traverse nodes
    # one by one
    one_node = head

    # To traverse nodes by
    # skipping one
    two_node = head

    # To keep track of previous middle
    prev = None

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

        # For previous node of middle node
        prev = one_node

        # Move one node each time
        one_node = one_node.next

        # Move two nodes each time
        two_node = two_node.next.next

    # Set middle node at head
    prev.next = prev.next.next
    one_node.next = head
    head = one_node

    # Return the modified head
    return head

def push(head, new_data):

    # Allocate new node
    new_node = Node(new_data)

    #Link the old list to new node
    new_node.next = head

    # Move the head to point the new node
    head = new_node

    # Return the modified head
    return head

# A function to print a given linked list
def printList(head):
    temp = head
    while (temp!=None):

        print(str(temp.data), end = " ")
        temp = temp.next
    print("")

# Create a list of 5 nodes
head = None
for i in range(5, 0, -1):
    head = push(head, i)

print(" list before: ", end = "")
printList(head)

head = setMiddleHead(head)

print(" list After: ", end = "")
printList(head)
# This code is contributed by Pranav Devarakonda

Output:

list before: 1 2 3 4 5
list After : 3 1 2 4 5 

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

Space Complexity: O(1) since using constant space

Please refer complete article on Make middle node head in a linked list for more details!