题解 | #链表中环的入口结点#

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # first check if it has a loop or not
        # if it has a loop, return the first position fast meet the slow
        # else return None
        def hascycle(head):
            if not head:
                return
            fast = slow = head
            while fast and fast.next:
                fast = fast.next.next
                slow = slow.next
                if fast == slow:
                    return fast
            return

        # slow1 at the head and the slow2 at the first meeting position
        # the next time they meet is the position where the ring started
        slow1 = hascycle(pHead)
        if not slow1:
            return
        else:
            slow2 = pHead
            while slow2 != slow1:
                slow1 = slow1.next
                slow2 = slow2.next
            return slow2