0142-linked-list-cycle-ii.js

/**
 * 142. Linked List Cycle II
 * https://leetcode.com/problems/linked-list-cycle-ii/
 * Difficulty: Medium
 *
 * Given the head of a linked list, return the node where the cycle begins.
 * If there is no cycle, return null.
 *
 * 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 (0-indexed). It is -1 if there is no cycle. Note that pos is
 * not passed as a parameter.
 *
 * Do not modify the linked list.
 */

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

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