如何使用双向链表在另一个节点之后插入一个节点？

Question

I want to insert a node after another node in a doubly-linked list but when I provide such a key that doesn't exist in the Linked list then this method gives me an error. How should I fix this error? here is my code. Thanks in Advance.

{
   public void AddAfter(int insertAfter, int data) 
   {
   DLNode n = new DLNode(data);
   DLNode curr = head;
   if(head == null)
       head = n;
   else
   {
       while(curr.Data != insertAfter && curr != null) 
       {
           curr = curr.next;
       }
       if(curr != null) 
       {
           n.next = curr.next;
           n.prev = curr;
           n.next.prev = n;
           curr.next = n;
           NumNodes++;
           
           
       }
       else
           System.out.println("The Key "+insertAfter+" doesn't exist: ");
   }
  }
}

Answer 1

... but when I provide such a key that doesn't exist in the Linked list then this method gives me an error.

The problem occur in your while loop condition (eg, curr.Data != insertAfter && curr != null ). Here, you are first accessing the data field of the curr variable and then checking whether curr is null or not.

How should I fix this error?

You need to do it in reverse order. The following code should solve your problem.

public void AddAfter(int insertAfter, int data) {
    DLNode n = new DLNode(data);
    DLNode curr = head;
    
    if(head == null) {
        head = n;
    }
    else {
        while(curr != null && curr.Data != insertAfter) {
            curr = curr.next;
        }
        if(curr != null) {
            n.next = curr.next;
            n.prev = curr;
            n.next.prev = n;
            curr.next = n;
            NumNodes++;
        }
        else {
            System.out.println("The Key "+insertAfter+" doesn't exist: ");
        }
    }
}