# 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.
"""
if not head or not head.next or not head.next.next:
return head
start = head
pre ,tail = self.findTailNode(head)
nextStart = start.next
pre.next = None
start.next = tail
tail.next = self.reorderList(nextStart)
return start
def findTailNode(self,head):
pre = head
tail = pre.next
while tail.next:
pre = tail
tail = tail.next
return pre ,tail
def creatList(self, l):
dummyHead = ListNode(0)
pre = dummyHead
for i in l:
pre.next = ListNode(i)
pre = pre.next
return dummyHead.next
def printList(self, head):
cur = head
while cur:
print cur.val
cur = cur.next
# 思路：
#
# 在中间平均截开两段，若为奇数，则第一段多一个
#
# 将第二段reverse
#
# 然后两段再交错地link起来
class Solution2(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: void Do not return anything, modify head in-place instead.
"""
if head is None or head.next is None or head.next.next is None:
head = head
else:
slow = fast = head
while fast.next and fast.next.next:
slow = fast.next
fast = fast.next.next
head2 = slow.next
slow.next = None
dummy = ListNode(0)
dummy.next = head2
p = head2.next
head2.next = None
while p:
# nextP = p.next
# p.next = dummy.next
# dummy.next = p
# p = nextP
tmp = p
p = p.next
tmp.next = dummy.next
dummy.next = tmp
head2 = dummy.next
p1 = head
p2 = head2
while p2:
t1 = p1.next
p1.next = p2
t2= p2.next
p2.next = t1
p1 = t1
p2 = t2
class Solution3(object):
def reorderList(self, head):
"""
:type head: ListNode
:rtype: void Do not return anything, modify head in-place instead.
"""
if head is None or head.next is None or head.next.next is None:
head = head
else:
slow = fast = head # two parts
while fast.next and fast.next.next:
slow = slow.next
fast = fast.next.next
head2 = slow.next
slow.next = None
dummy = ListNode(0) # reverse 2nd part
dummy.next = head2
p = head2.next
head2.next = None
while p:
tmp = p
p = p.next
tmp.next = dummy.next
dummy.next = tmp
head2 = dummy.next
p1 = head # rejoin 2 parts together
p2 = head2
while p2:
t1 = p1.next
p1.next = p2
t2 = p2.next
p2.next = t1
p1 = t1
p2 = t2
l1 = [1,2,3,4,5,6]
s = Solution()
head = s.creatList(l1)
s.printList(head)
head = s.reorderList(head)
s.printList(head)
