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