# -*- coding:utf-8 -*-

# from numpy import heaviside
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        if pHead is None:
            return None

        slow = pHead
        fast = pHead
        while fast != None and fast.next != None:
            fast = fast.next.next
            slow = slow.next
            if fast == slow: # 判断是否有环
                fast = pHead # 快指针回到链表头
                while fast != slow: # 分别一步一步遍历
                    fast = fast.next 
                    slow = slow.next
                return slow # 环入口
                    
        return None

主要思路

• 先判断链表是否有环，找到相遇的节点。
• 快指针回到链表头，慢指针继续在相遇节点，两个指针同步逐个元素遍历链表。
• 再次相遇的地方就是环的入口。