题解 | #链表中环的入口结点#

from heapq import heapify

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        node = self.hasCycle(pHead)
        p = pHead
        if node:
            while p != node:
                p = p.next
                node = node.next
            return p
        else:
            return None

    def hasCycle(self, pHead):
        # write code here
        fast = pHead
        slow = pHead
        while fast and slow:
            if fast and fast.next:
                slow = slow.next
                fast = fast.next.next
            else:
                return None
            if fast == slow:
                return slow
        return None

环问题：如何找到环的起始点。先找到相遇点 让快慢指针同时分别从头结点和相遇点以相同的速度出发，再次相遇点即为起点。