/**
 * 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}
 */
const middleNode = function (head) {

};

function ListNode(val, next) {
  this.val = (val === undefined ? 0 : val);
  this.next = (next === undefined ? null : next);
}

/*
如果利用快慢指针寻找重点，如果有两个中点返回后面那个，直接利用靠右的判断方法 fast && fast.next
*/
function f1(head) {
  let slow = head;
  let fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
  }
  return slow;
}