class Node {
    int data;
    Node next;

    public Node(int data, Node next) {
        this.data = data;
        this.next = next;
    }
}

class IntegerLinkedList {
    Node front;

    IntegerLinkedList() { front = null; }

    /** @return true iff the list is empty */
    boolean isEmpty() { return front == null; }

    /**
     * Inserts a new node in front of the list.
     * @param aNode a Node to be inserted into the list
     */
    void insert(Node aNode) {
        aNode.next = front;
        front = aNode;
    }

    /**
     * Deletes the node in front of the list.
     * @return the value of the deleted node
     */
    int delete() {
        if (isEmpty())
            throw new RuntimeException("Trying to delete an empty list.");
        int val = front.data;
        front = front.next;
        return val;
    }

    /**
     * @return the value of the data in the node at front of the list
     */
    int peek() {
        if (isEmpty())
            throw new RuntimeException("Trying to peek at an empty list.");
        return front.data;
    }

    /** Returns a string representation of the list. */
    public String toString() {
        StringBuilder sb = new StringBuilder();
        for (Node node = front; node != null; node = node.next) {
            sb.append(" " + node.data);
        }
        return sb.toString();
    }
}