Manjusaka

09-04 leetcode-0141

链接 141. Linked List Cycle

题目

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.

题解

经典题

from typing import Optional


class ListNode:
    def __init__(self, x: int):
        self.val = x
        self.next: Optional["ListNode"] = None


class Solution:
    def hasCycle(self, head: Optional["ListNode"]) -> bool:
        first = head
        second = head
        while True:
            if not first or not first.next:
                return False
            first = first.next
            if not second or not second.next or not second.next.next:
                return False
            second = second.next.next
            if first == second:
                return True