0142_linked_list_cycle_ii_(medium).js

/**
 * 142. Linked List Cycle II
 *
 * https://leetcode.com/problems/linked-list-cycle-ii/
 *
 * Given a linked list, return the node where the cycle begins. If there is no cycle, return null.
 *
 * Note: Do not modify the linked list.
 *
 * Follow up:
 * Can you solve it without using extra space?
 */

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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
const detectCycle = (head) => {
  let slow = head;
  let fast = head;

  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;

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

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

      return slow;
    }
  }

  return null;
};

export { detectCycle };