题解 | #链表中环的入口结点#

双指针的python实现：

# -*- coding:utf-8 -*-
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None
class Solution:
    def EntryNodeOfLoop(self, pHead):
        # write code here
        fast, slow = pHead, pHead
        while True:
            if not (fast and fast.next): return
            fast, slow = fast.next.next, slow.next
            if fast == slow: break
        fast = pHead
        while fast!=slow:
            fast, slow = fast.next, slow.next
        return fast