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141.py
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141.py
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# 141. Linked List Cycle
# Given a linked list, determine if it has a cycle in it.
# To represent a cycle in the given linked list,
# we use an integer pos which represents the position (0-indexed) in the linked list where tail connects to.
# If pos is -1, then there is no cycle in the linked list.
# Example 1:
# Input: head = [3,2,0,-4], pos = 1
# Output: true
# Explanation: There is a cycle in the linked list, where tail connects to the second node.
# Example 2:
# Input: head = [1,2], pos = 0
# Output: true
# Explanation: There is a cycle in the linked list, where tail connects to the first node.
# Example 3:
# Input: head = [1], pos = -1
# Output: false
# Explanation: There is no cycle in the linked list.
# Follow up:
# Can you solve it using O(1) (i.e. constant) memory?
# Definition for singly-linked list.
# class ListNode(object):
# def __init__(self, x):
# self.val = x
# self.next = None
class Solution(object):
def hasCycle(self, head):
"""
:type head: ListNode
:rtype: bool
"""
if not head or not head.next:
return False
slow = head
fast = head.next
while slow != fast:
if not fast or not fast.next:
return False
fast = fast.next.next
slow = slow.next
return True