[Neetcode/Linked List] Linked List Cycle Detection

https://neetcode.io/problems/linked-list-cycle-detection?list=neetcode150

NeetCode

Linked List (토끼와 거북이 경주 -> 속도가 다르더라도 결국 만나게 된다!)

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

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