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):
        # write code here
        # 判断链表是否有环，使用快慢指针的方式进行判断
        slow = self.ishuan(pHead)
        if slow==None:
            return None
        fast = pHead     
        while fast != slow:  # 第二次相关的循环判断节点
            # 链表不为空
            slow = slow.next
            if fast.next:
                fast = fast.next
            else:
                return None
        return slow
        
    def ishuan(self, pHead):
        fast = pHead
        slow = pHead
        if not pHead:
            return pHead
        while slow and fast:
            # 链表不为空
            slow = slow.next
            if fast.next:
                fast = fast.next.next
            else:
                return None

            if slow==fast: 
                return slow # 快慢指针第一次相遇，判断是否有环出现，这时需要固定slow进行遍历
        return None