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reorder-list.py
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reorder-list.py
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'''
Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
You must do this in-place without altering the nodes' values.
For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.
'''
class Solution(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: void Do not return anything, modify head in-place instead.
"""
if not head or not head.next or not head.next.next:
return
slow = head
fast = head
while fast and fast.next: # find middle node
slow = slow.next
fast = fast.next.next
pre = None
current = slow.next # slow may not exists
slow.next = None
while current: reverse latter half list
temp = current
current = current.next
temp.next = pre
pre = temp
pos = head
neg = pre
while neg:
temp = neg
neg = neg.next
temp.next = pos.next
pos.next = temp
pos = temp.next