剑指Offer:链表中环的入口结点

思路：

Python：

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        if not pHead:
            return None
        fast=pHead
        slow=pHead
        while fast and fast.next:
            slow=slow.next
            fast=fast.next.next
            if slow==fast:
                break
        if not fast or not fast.next:
            return None
        fast=pHead
        while fast!=slow:
            fast=fast.next
            slow=slow.next
        return fast