leetcode 143. Reorder List

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题目链接：leetcode

思路：
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# 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:return None
        slow = node = head
        fast = slow.next
        stack=[]
        while fast and fast.next:
            slow = slow.next
            fast = fast.next.next
        new_slow = slow.next
        slow.next = None
        while new_slow:
            stack.append(new_slow)
            new_slow = new_slow.next
        while node:
            next_node = node.next
            if stack:
                node.next=stack.pop()
                node.next.next = next_node
            node = next_node