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.

Example

Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.

Solution

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

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var detectCycle = function (head) {
  // If head is empty return null
  if (!head) return null;
  if (!head.next) return null;

  // Set slow and fast to head
  let slow = head;
  let fast = head;
  let pointer = head;

  // While fast and fast.next are not null
  while (fast && fast.next) {
    fast = fast.next.next;
    slow = slow.next;
    if (fast === slow) break;
  }

  if (fast !== slow) return null;

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

  return slow;
};