# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def detectCycle(self, head):
"""
:type head: ListNode
:rtype: ListNode
"""
if not head or not head.next:
return
slow = head
fast = head
slow = slow.next
fast = fast.next.next
while fast and fast.next:
slow = slow.next
fast = fast.next.next
if fast == slow:
slow = head
# Lslow=K+M;Lfast=K+M+n*L;Lfast=2*Lslow。
# 得出 Lslow=n*L;K=n*L-M。
# slow重新回到head,而fast还在相遇点,slow和fast都向前走,且每次走一个节点。
# 则slow从head走到起点走了K,而fast从相遇点出发也走了K,由于K=(n-1)*L+(L-M),所以fast转了n-1圈,再走L-M,也到了起点。
while slow != fast:
slow = slow.next
fast = fast.next
return slow
return
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