LeetCode设计链表代码Bug排查：删除操作异常求助

The core issue causing your test case failure lies in the addAtIndex function: when inserting a new node, you're not linking it to the subsequent node in the list, which breaks the chain and loses the tail portion of the list.

Root Cause #

In your current addAtIndex implementation, after creating the new node, you directly set iter?.next = new but never assign new.next to the original iter?.next. This means the new node's next remains null, cutting off the rest of the list. For your test case, inserting 2 at index 1 breaks the link between 1 and 3, so after insertion, the list becomes 1 -> 2 instead of 1 -> 2 -> 3. When you later delete index 1, the list is just 1, and trying to get index 1 (which is out of bounds for size 1) returns -1.

Fixed Code for addAtIndex #

fun addAtIndex(index: Int, `val`: Int) {
    if (index > size)
        return

    if (index == 0) {
        addAtHead(`val`)
        return
    }
    
    var iter = head
    for (i in 0 until index - 1) {
        iter = iter?.next
    }
    val new = ListNode(`val`)
    // Fix: Link new node to the next node before updating iter's next
    new.next = iter?.next
    iter?.next = new
    size++
}

Additional Minor Bug Fix (addAtTail for Empty List) #

Your addAtTail function doesn't handle the case when the list is empty (size=0). Adding this check ensures proper behavior:

fun addAtTail(`val`: Int) {
    if (size == 0) {
        addAtHead(`val`)
        return
    }
    var iter = head
    for (i in 0 until size - 1) {
        iter = iter?.next
    }
    
    val new = ListNode(`val`)
    iter?.next = new
    size++
}

Verification with Your Test Case #

After applying the fix:

1. addAtHead(1) → List: 1, size=1
2. addAtTail(3) → List: 1->3, size=2
3. addAtIndex(1,2) → List: 1->2->3, size=3
4. get(1) returns 2 (correct)
5. deleteAtIndex(1) → List: 1->3, size=2
6. get(1) returns 3 (matches expected output)

内容的提问来源于stack exchange，提问作者Aleksandr T