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"""
# 141. Linked List Cycle
- https://leetcode.com/problems/linked-list-cycle/
- Classification: Fast & Slow pointers, Floyd's Algorithm
## Challenge
Given head, the head of a linked list,
determine if the linked list has a cycle in it.
There is a cycle in a linked list if there is some node
in the list that can be reached again by continuously
following the next pointer.
Internally, pos is used to denote the index of the node that
tail's next pointer is connected to.
Note that pos is not passed as a parameter.
Return true if there is a cycle in the linked list.
Otherwise, return false.
## Solution
Floyd's algorithm.
Using fast and slow pointers.
If fast and slow pointers meet there is proof of a loop.
If the fast pointer runs out there is no cicle
This can be implemented with a while fast and fast.next
or a try except
"""
# Definition for singly-linked list.
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
# Solution 1
def hasCycle(self, head: ListNode) -> bool:
slow = fast = head
while fast and fast.next:
fast = fast.next.next
slow = slow.next
if fast == slow: return True # pointers met: a loop detected
return False # fast pointer reached the end: no loop
#Solution 1, variant with try except
def hasCycle(self, head: ListNode) -> bool:
slow = fast = head
try:
while True:
fast = fast.next.next
slow = slow.next
if slow == fast: return True
except: # .next was None
return False