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问题描述：

• Given a linked list, determine if it has a cycle in it.

思路：

• 采用快慢指针法，快指针每次前进两个节点，慢指针每次前进一个节点

• 无环条件：node.next=NULL

• 有环条件：快指针指向的节点=慢指针指向的节点

代码：

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = Noneclass Solution(object):def hasCycle(self, head):""":type head: ListNode:rtype: bool"""slow=fast=headwhile True:try:fast=fast.next.nextslow=slow.nextif fast==slow:return Trueexcept:return False

结果：

Runtime: 40 ms, faster than 100.00% of Python online submissions for Linked List Cycle.