给一个链表，若其中包含环，请找出该链表的环的入口结点，否则，输出null。

class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        if  not pHead:
            return None
        start = pHead
        dic = {}
        while start:
            if start in dic:
                return start
            else:
                dic[start] = start.val
            start = start.next
        return None