141-linked-list-cycle.js

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
const hasCycle = function(head) {
  const seen = []
  while(head != null) {
        if(seen.indexOf(head) !== -1) {
          return true
        } else {
          seen.push(head)
        }
        head = head.next
  }
  return false
};