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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 head == None or head.next == None:
            return
        fast = head
        slow = head
        while fast.next != None and fast.next.next != None:
            fast = fast.next.next
            slow = slow.next
        head2 = slow.next
        slow.next = None
        head2 = self.reverse(head2)
        self.merge(head, head2)
    def reverse(self, node):
        dummy = ListNode(0)
        dummy.next = node
        p = node
        # dummy->..-p->curNode->cur.next..
        # dummy->curNode->..->p->cur.next..
        while p.next != None:
            curNode = p.next
            p.next = curNode.next
            curNode.next = dummy.next
            dummy.next = curNode
        return dummy.next
    def merge(self, head, head2):
        p = head
        head = head.next
        while head and head2:
            # Insert a head2 node.
            p.next = head2
            head2 = head2.next
            p = p.next
            # Insert a head node.
            p.next = head
            head = head.next
            p = p.next
        if head == None:
            p.next = head2
        if head2 == None:
            p.next = head