空间复杂度o(1)，时间复杂度o(n)
从head开始，每经过一个节点便截断，过河拆桥拆到最后就是环的起点了

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        a = ListNode('')
        while pHead:
            if pHead.next == a:
                return pHead
            tmp = pHead.next
            pHead.next = a
            pHead = tmp