【LeetCode】142.环形链表 II

#!/usr/bin/python3
# -*- coding: utf-8 -*-
# @Time: 2019/3/26
# @Author: xfLi
# The file...

"""
问题分析：
利用set来存放已经遍历的节点
"""

def hasCycle(head):
    if not head:
        return None
    res = set()
    cur = head
    while cur:
        if cur in res:
            return cur
        else:
            res.add(cur)
            cur = cur.next
    return None