[876] 链表的中间结点

/*
 * @lc app=leetcode.cn id=876 lang=javascript
 *
 * [876] 链表的中间结点
 */

// @lc code=start
/**
 * 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 slow = head
  let fast = head
  // 快指针走两步，慢指针走一步，当快指针走到边界是慢指针当前的位置就是中间点
  while (fast !== null && fast.next !== null) {
    slow = slow.next
    fast = fast.next.next
  }
  return slow
}
// @lc code=end