dummy = ListNode()
    dummy.next = head
	prev = dummy
	while head:
		if hashmap[head.val] > 1:
			prev.next = head.next
		else:
			prev = head
		head = head.next
	return dummy.next

	prev = None 
	while head:
		next = head.next
		head.next = prev
		prev = head
		head = next
	return prev

1.  find the rightmost not-nine digit
2.  increase this rightmost not-nine digit by 1
3.  set all the following nines to zeros

fast_pointer = head
slow_pointer = head
while fast_pointer or fast_pointer.next:
	fast_pointer = fast_pointer.next.next
	slow_pointer = slow_pointer

快慢指针主要用来找到链表的中间点， 如果链表的长度是奇数时，慢指针最终指向链表的中间点， 如果链表的长度是偶数时，慢指针最终指向第（n/2 + 1） 个点（例如链表长度是4，慢指针最终指向第三个点）
判断链表是否有环也可以使用快慢指针。 如果有环， 则快指针与慢指针最终会相遇。

遇到two pointer 的题， 如果从前后双指针，可以先使用快慢指针确定中间节点， 然后翻转后半段链表。这样就可以前后同时进行遍历了

When we would like to construct an integer from a string or linked list, we can always use this pattern. For example: 
string = 'xxx'
integer = 0
for item in string:
	integer = integer*base + item
Here base can be 10 base, 2 base.