0141-linked-list-cycle.js

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

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

/**
 * @param {ListNode} head
 * @return {boolean}
 */
var hasCycle = function(head) {
  while (head) {
    if (head.visited) {
      return true;
    }
    head.visited = 1;
    head = head.next;
  }

  return false;
};