题目描述
给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def EntryNodeOfLoop(self, pHead):
# write code here
head = ListNode(0)
head.next = pHead
fast = head
slow = head
while True:
if fast and fast.next:
fast = fast.next.next
slow = slow.next
else:
return None
if fast == slow:
fast = head
break
while fast != slow:
fast = fast.next
slow = slow.next
return fast