141. Linked List Cycle

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

JavaScript

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
  let l1 = new ListNode()
  let l2 = new ListNode()
  l1.next = l2.next = head

  while (l1 !== l2) {
    if (!l1 || !l2 || !l2.next) {
      return false
    }
    l1 = l1.next
    l2 = l2.next.next
  }

  return true
}

Python

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


class Solution(object):

    def hasCycle(self, head):
        """
        :type head: ListNode
        :rtype: bool
        """
        l1 = ListNode(None)
        l2 = ListNode(None)
        l1.next = l2.next = head
        while l1 != l2:
            if not l1 or not l2 or not l2.next:
                return False
            l1 = l1.next
            l2 = l2.next.next

        return True