add: 链表的中间节点

test/list/linked-list-cycle.test.js

@@ -1,20 +1,5 @@
 import { hasCycle } from '../../src/list/linked-list-cycle'
-
-function ListNode (val) {
-  this.val = val
-  this.next = null
-}
-
-const arrToList = (arr) => {
-  const head = new ListNode(arr[0])
-  let current = head
-  for (let n = 1, len = arr.length; n < len; n++) {
-    current.next = new ListNode(arr[n])
-    current = current.next
-  }
-
-  return head
-}
+import { arrToList } from './ListNode'
 
 test('环形链表', () => {
   const list = arrToList([1, 2, 3])