其实最初有点懵逼，这种题老是读不懂，其实就是找重复节点


# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        stack = []
        while pHead:
            stack.append(pHead)
            pHead = pHead.next
            if pHead in stack:
                return pHead
        return None