leetcode-876-middleOfTheLinkedList.js

/**
 * 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}
 */

// p: head of ll;
// r: node of ll;
// e:
//          f
// [1,2,3,4,5]
//      s
//               f
// [1,2,3,4,5,6]
//        s
// s
// 1
// f
var middleNode = function (head) {
  let s = head,
    f = head;
  while (f && f.next) {
    s = s.next;
    f = f.next.next;
  }
  return s;
};