# 143. Reorder List
"""
given the head of a singly linked list. the list can be represented as 'l0 ->
l1 -> ... -> Ln'. reorder the list to be of the following form: 'l0 -> ln ->
ln-1 -> ln-2'. you may not modify the values in the list's nodes. only nodes
themselves may be changed.
"""
# Definition for singly-linked list.
class ListNode(object):
def __init__(self, val=0, next=None):
self.val = val
self.next = next
class Solution(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: None Do not return anything, modify head in-place instead.
"""
if not head:
return []
slow, fast = head, head
while fast.next and fast.next.next:
slow = slow.next
fast = fast.next.next
prev, curr = None, slow.next
while curr:
tmp = curr.next
curr.next = prev
prev = curr
curr = tmp
slow.next = None
head1, head2 = head, prev
while head2:
tmp = head1.next
head1.next = head2
head1 = head2
head2 = tmp
