在LinkedList中实现AddAtIndex方法

class ListNode {
    int val;
    ListNode next;
    
    public ListNode(int val) {
        this.val = val;
    }
}

class LinkedList {
    private ListNode head;
    
    public void addAtIndex(int index, int val) {
        if (index < 0) {
            return;
        }
        
        ListNode newNode = new ListNode(val);
        
        if (index == 0) {
            newNode.next = head;
            head = newNode;
            return;
        }
        
        ListNode prev = head;
        for (int i = 0; i < index - 1; i++) {
            if (prev == null) {
                return;
            }
            prev = prev.next;
        }
        
        newNode.next = prev.next;
        prev.next = newNode;
    }
}