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 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!