142.js

/** ------------------------------------------------------------------------------
 * 
 * 2023-03-09
 * 142. Linked List Cycle II
 * https://leetcode.com/problems/linked-list-cycle-ii/description/
 * 
 ------------------------------------------------------------------------------ */
/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function (head) {
  let p1 = head,
    p2 = head

  while (p2 != null && p2.next != null) {
    p1 = p1.next
    p2 = p2.next.next
    if (p1 === p2) break
  }

  if (p2 === null || p2.next === null) return null
  while (head != p1) {
    head = head.next
    p1 = p1.next
  }
  return head
}

console.log(detectCycle([1, 2]))

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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function (head) {
  let fast = head
  let slow = head

  while (fast && fast.next !== null) {
    slow = slow.next
    fast = fast.next.next

    if (fast === slow) {
      slow = head

      while (slow !== fast) {
        slow = slow.next
        fast = fast.next
      }

      return slow
    }
  }

  return null
}