双链列表的加法

Question

I realize this should be a very easy method to implement and I have no idea why this code is not working.

I get a NullPointerException at tail.next = node . If it is not commented out, it does not pass the tester.

public void addBack (int x)
{
    IntegerNode node = new IntegerNode(x,null);
    if (head == null)
        head = node;
    else{
        node.prev = tail;
        //tail.next = node;
        tail = node;

    }

    count++;
}

Answer 1

When the list is empty, set your head and tail to the new element. Should be ok after that, though more null checks are rarely a bad idea.

if (head == null) {
    head = node;
    tail = node;
} else {
    node.prev = tail;
    tail.next = null; // Did you really mean tail.next = node?  This looks more likely..
    tail = node;
}