优化 707设计链表 addAtIndex

problems/0707.设计链表.md

@@ -917,16 +917,17 @@ class MyLinkedList {
     }
 
     fun addAtIndex(index: Int, `val`: Int) {
-        if (index == 0) return addAtHead(`val`)
-        if (index == size) return addAtTail(`val`)
         if (index > size) return
-        var cur = this.next
-        for (i in 0 until index - 1) {
+        val pre = ListNode(0)
+        pre.next = this.next
+        var cur:ListNode? = pre
+        for (i in 0 until index) {
             cur = cur?.next
         }
         val temp = cur?.next
         cur?.next = ListNode(`val`)
         cur?.next?.next = temp
+        this.next = pre.next
         size++
     }