# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    
    #定义全局变量
    def __init__(self):
        #标识三个相邻的节点
        self._1 = None
        self._2 = None
        self._3 = None

        #用于返回新链表的哨兵头节点        
        self.newListHead = ListNode(None)
        self.newListHead.next = None
        
        #用于生成新链表的当前节点
        self.curListHead = self.newListHead
        
    #根据 self._2 的值，为新链表中增加一个节点
    def genOneNodeToList(self):
        print self._2
        newNode = ListNode(self._2)
        newNode.next = None
        
        self.curListHead.next = newNode
        self.curListHead = self.curListHead.next
    
    #检查当前节点 self._2 数据和前后节点是否相同
    #如果分别不同与前后节点，则不重复
    def duplicate(self, val):
        self._3 = self._2
        self._2 = self._1
        self._1 = val
        if self._2 != self._1 and self._2 != self._3:
            self.genOneNodeToList()
        
    def deleteDuplicates(self, head):
        """
        :type head: ListNode
        :rtype: ListNode
        """
        iterHead = head
        while True:
            if iterHead != None:
                self.duplicate(iterHead.val)
            else:
                self.duplicate(None)
                break
            iterHead = iterHead.next
            
        return self.newListHead.next