key points:
-simultaneous assignment
-dummy head node to thread linked lists
-use prev node to reverse linked list
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: void Do not return anything, modify head in-place instead.
"""
#divide the list into two halves, prev is the node before the starting node of second list
if head is None or head.next is None: return
slow,fast,prev=head,head,None
while fast and fast.next :
prev,slow,fast=slow,slow.next,fast.next.next
#reverse the second list
current,prev.next,prev=slow,None,None
while current:
current.next,prev,current=prev,current,current.next
#create a dummy head and thread two list one by one
l1,l2=head,prev
dummy=ListNode(0)
current=dummy
while l1!=None and l2!=None:
current.next,current,l1 =l1,l1,l1.next
current.next,current,l2 =l2,l2,l2.next
143. Reorder List
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