1 # -*- coding:utf-8 -*-
2 # class ListNode:
3 # def __init__(self, x):
4 # self.val = x
5 # self.next = None
6 class Solution:
7 def EntryNodeOfLoop(self, pHead):
8 # write code here
9 #先判断是不是有环，用快慢指针（一个正常走，一个每次走两步），如果有环必然会相遇
10 if pHead == None:
11 return None
12 meetingnode = self.MeetingNode(pHead)
13 if meetingnode == None:#如果判断无交点的的情况
14 return None
15 nodeslop = 1
16 node1 = meetingnode #如果有交点，返回交点就够了。环的节点数比运行次数多1
17 while node1.next != meetingnode:
18 node1 = node1.next
19 nodeslop += 1
20 node1 = pHead #原链表一直没动
21 for i in range(nodeslop):#两个指针开始走，当先让一个指针走环内节点数的步数时，另一个节点再走，此时相遇的点恰好是成环第一个节点
22 node1 = node1.next
23 node2 = pHead
24 while node1 != node2:
25 node1 = node1.next
26 node2 = node2.next
27 return node1
28
29 def MeetingNode(self, pHead):
30 slow = pHead.next
31 if slow == None:
32 return None
33 fast = slow.next
34 while fast != None:
35 if slow == fast:
36 return fast
37 slow = slow.next
38 fast = fast.next
39 if fast != None: #不是循环只会运行一次
40 fast = fast.next
41 return None
