141. Linked List Cycle

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

Python

Floyd's Tortoise and Hare Algorithm

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

        while is_first or slow != fast:
            is_first = False

            if not fast or not fast.next:
                return False
            slow = slow.next
            fast = fast.next.next

        return True

Javascript

var hasCycle = function(head) {  
    var slow = head;
    var fast = head;
  
    while (fast !== null && fast.next !== null) {      
        slow = slow.next;
        fast = fast.next.next;
      
        if (slow === fast) return true;              
    }
    return false;
};