leetcode/work/1/Easy/Hash Table/141.linked-list-cycle.py

"""
141. Linked List Cycle
Difficulty: Easy
https://leetcode.com/problems/linked-list-cycle/

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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, 10^4].

        • -10^5 <= Node.val <= 10^5

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



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

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None


class Solution:
    def hasCycle(self, head: Optional[ListNode]) -> bool:
        fast = slow = head

        while fast and fast.next:
            fast = fast.next.next
            slow = slow.next
            if fast is slow:
                return True
        return False