LinkedListCycle

Problem

https://leetcode.com/problems/linked-list-cycle/

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.

Example 1:

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Constraints:

• The number of the nodes in the list is in the range [0, 10:sup:`4`].

• -10:sup:`5`<= Node.val <= 10:sup:`5`

• pos is -1 or a valid index in the linked-list.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

Solution

Set two pointers, a and b, to head. Move a 1 node at a time but move b 2 nodes at a time. If a and b ever point to the same node, then b must have wrapped around the linked list and caught up to a.

Pattern

Hash Table, Linked List, Two Pointers

Code

from __future__ import annotations

from typing import List


class ListNode:
    """Node in a linked list.
    """

    def __init__(self, val: int, next: ListNode | None = None):
        self.val = val
        self.next = next

    @classmethod
    def from_list(cls, list: List[int]) -> ListNode | None:
        head: ListNode | None = None
        for el in list:
            if head is None:
                head = ListNode(el)
                node = head
            else:
                node.next = ListNode(el)
                node = node.next
        return head


def hasCycle(head: ListNode | None) -> bool:
    """Whether the linked list has a cycle.
    """
    if head is None:
        return False

    a = head
    b = head.next

    while b is not None:
        if a is b:
            return True

        a = a.next
        b = b.next
        if b is not None:
            b = b.next

    return False

Test

>>> from LinkedListCycle import ListNode, hasCycle
>>> head = ListNode.from_list([3, 2, 0, -4])
>>> head.next.next.next.next = head.next
>>> hasCycle(head)
True
>>> head = ListNode.from_list([1, 2])
>>> head.next.next = head
>>> hasCycle(head)
True
>>> head = ListNode.from_list([1])
>>> hasCycle(head)
False

class LinkedListCycle.ListNode(val: int, next: ListNode | None = None)

Bases: object

Node in a linked list.

classmethod from_list(list: List[int]) → ListNode | None

LinkedListCycle.hasCycle(head: ListNode | None) → bool

Whether the linked list has a cycle.