/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var middleNode = function (head) {
  let cur = head;
  let cnt = 1;
  while (cur.next) {
    cur = cur.next;
    cnt++;
  }
  cnt = parseInt(cnt / 2);
  while (cnt) {
    head = head.next;
    cnt--;
  }
  return head;
};

console.log(middleNode([1, 2, 3, 4, 5]));

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */
 var middleNode = function(head) {
  let fast = head;
  let slow = head;

  // during each traversal, we'll move
  // fast at 2 increments, and slow at 1 increment
  while (fast && fast.next) {
    fast = fast.next.next;
    slow = slow.next;
  }

  // by the time we finish the while loop, slow pointer
  // should point to the middle node;
  return slow;
};