链表中的双/多指针问题，环的问题
一、Find the Middle Node
1 def find_middle(lst): 2 assert lst.head is not None and lst.head.next is not None 3 4 head = lst.head 5 fast = head 6 slow = head 7 8 while fast is not None and fast.next is not None: 9 fast = fast.next.next 10 slow = slow.next 11 12 return slow.value