Using a stack to store treap nodes when adding new nodes. Why am I getting an EmptyStackException?

Question

I'm building a treap class in Java. Below is my function for adding new nodes to the treap. The process is: traverse down to bottom of treap (while adding each node in the path to a local stack) only worrying about the BST structure at first, then, once at the bottom, I'm reestablishing the heap invariant through rotations by utilizing the stack I built.

It sounds like it should work, but I keep getting an EmptyStackException. This exception occurs once the private "reheap" function is called.

The add function works for the first node added to the treap, such as if I did: testTree.add (4 ,19); But fails upon the second addition of a node, like if I then call: testTree.add (2 ,31);

This is the full error:

Exception in thread "main" java.util.EmptyStackException
    at java.util.Stack.peek(Unknown Source)
    at classes.Treap.reheap(Treap.java:137)
    at classes.Treap.add(Treap.java:130)
    at classes.Treap.main(Treap.java:220)

130 -- refers to the call to reheap at the bottom of the add function 137 -- refers to the while loop of the private reheap function 220 -- refers to my attempt to add the new node in main.

I've tried changing around the conditionals in the reheap function, but to no avail.

boolean add(E key, int priority) {
        Stack<Node<E>> stack = new Stack<Node<E>>();
        if(root == null) {
            Node<E> newroot = new Node<E>(key, priority);
            root = newroot;
            stack.push(root);
            return true;
        }else {

            Node<E> current = new Node<E>(root.data, root.priority); //placeholder, used for traversing
            Node<E> added = new Node<E>(key, priority);  //node to be added to the treap
            if(this.find(key) == true){
                return false;
            }else {
                if(current.right == null && current.left == null) {
                    stack.push(current);
                    if(key.compareTo(current.data) < 0)
                        current = current.left;
                    else
                        current = current.right;
                }
                else {
                    while(current.right != null || current.left != null) {
                        if(key.compareTo(current.data) < 0) {
                            stack.push(current);
                            current = current.left;
                        }
                        if(key.compareTo(current.data) > 0) {
                            stack.push(current);
                            current = current.right;
                        }
                    }
                }
                if(key.compareTo(stack.peek().data) < 0)
                    stack.peek().left = added;
                else if(key.compareTo(stack.peek().data) > 0)
                    stack.peek().right = added;
                if(!stack.isEmpty())
                    this.reheap(added, stack);
                return true;
            }
        }
    }

    private boolean reheap(Node<E> added, Stack<Node<E>> stack) {
        while(added.priority > stack.peek().priority && !stack.isEmpty()) {
            if(stack.peek().right == added)
                stack.peek().rotateLeft();
            else
                stack.peek().rotateRight();
            stack.pop();
        }
        return true;
    }

After the second call ( testTree.add (2 ,31); ), I should be getting a treap with structure ( Node(2, null, Node(4)) ). <-- this would be after the reheaping of course.

Answer 1

You need to do the empty stack check prior to peeking the stack!

while(!stack.isEmpty() && added.priority > stack.peek().priority) {...}