链表中环的入口结点

快慢指针f , s，两指针相遇时，f = 2* s, 设环长度为n，n = s
再一个慢指针从链表头开始，两个慢指针相遇的地方则为入口

# -*- 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.next:
            return None
        fast = pHead
        slow = pHead
        while fast.next and fast.next.next:
            fast = fast.next.next
            slow = slow.next
            if fast == slow:
                break
        slow2 = pHead
        while slow != slow2:
            slow = slow.next
            slow2 = slow2.next
        return slow