diff --git a/src/aud/exam/prep/tree/BinaryTreeIterator.java b/src/aud/exam/prep/tree/BinaryTreeIterator.java
new file mode 100644
index 0000000..1cdc816
--- /dev/null
+++ b/src/aud/exam/prep/tree/BinaryTreeIterator.java
@@ -0,0 +1,57 @@
+package aud.exam.prep.tree;
+
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+class BinaryTreeIterator<T> implements Iterator<T> {
+
+    private final Stack<StackFrame> stack = new Stack<>();
+
+    BinaryTreeIterator(BinaryTreeNode<T> node) {
+        if (node != null) {
+            var frame = new StackFrame();
+            frame.node = node;
+            stack.push(frame);
+        }
+    }
+
+    @Override
+    public boolean hasNext() {
+        return !stack.empty();
+    }
+
+    @Override
+    public T next() {
+        if (!hasNext()) {
+            throw new NoSuchElementException();
+        }
+
+        var frame = stack.peek();
+
+        while (!frame.pushedLeft) {
+            frame.pushedLeft = true;
+
+            if (frame.node.left != null) {
+                var newFrame = new StackFrame();
+                newFrame.node = frame.node.left;
+                stack.push(newFrame);
+                frame = newFrame;
+            }
+        }
+
+        stack.pop();
+
+        if (frame.node.right != null) {
+            var newFrame = new StackFrame();
+            newFrame.node = frame.node.right;
+            stack.push(newFrame);
+        }
+
+        return frame.node.key;
+    }
+
+    private class StackFrame {
+        boolean pushedLeft = false;
+        BinaryTreeNode<T> node;
+    }
+}
diff --git a/src/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessor.java b/src/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessor.java
new file mode 100644
index 0000000..b73ccc2
--- /dev/null
+++ b/src/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessor.java
@@ -0,0 +1,36 @@
+package aud.exam.prep.tree;
+
+abstract class OrderedBinaryTreeNodeProcessor<V> implements OrderedTreeProcessor<V, BinaryTreeNode<V>> {
+    @Override
+    public BinaryTreeNode<V> newEmptyTree() {
+        return null;
+    }
+
+    @Override
+    public Iterable<V> iterate(BinaryTreeNode<V> tree) {
+        return () ->
+                new BinaryTreeIterator<>(tree);
+    }
+
+    @Override
+    public void print(BinaryTreeNode<V> tree) {
+        printRec(tree, 0);
+    }
+
+    private void printRec(BinaryTreeNode<V> tree, int indent) {
+        if (tree == null) {
+            System.out.println("- *");
+            return;
+        }
+
+        System.out.println("-> " + tree.key);
+
+        System.out.print(" |".repeat(indent));
+        System.out.print(" L");
+        printRec(tree.left, indent + 1);
+
+        System.out.print(" |".repeat(indent));
+        System.out.print(" L");
+        printRec(tree.right, indent + 1);
+    }
+}
\ No newline at end of file
diff --git a/src/aud/exam/prep/tree/OrderedTreeProcessor.java b/src/aud/exam/prep/tree/OrderedTreeProcessor.java
new file mode 100644
index 0000000..4424124
--- /dev/null
+++ b/src/aud/exam/prep/tree/OrderedTreeProcessor.java
@@ -0,0 +1,82 @@
+package aud.exam.prep.tree;
+
+import java.util.Comparator;
+
+/**
+ * Instances of this interface can work with any "valid" tree of type <code>T</code>.
+ * For what is valid, see {@link #check(T, Comparator)}.
+ * For this processor trees always have to be "in order".
+ * This is a literal translation of the "inspiration sheet" into java.
+ *
+ * @param <V> Type of elements stored in <code>T</code>
+ * @param <T> Type of the tree
+ */
+public interface OrderedTreeProcessor<V, T> {
+
+    boolean find(T t, V v, Comparator<V> cmp);
+
+    boolean override(T t, V from, V to, Comparator<V> cmp);
+
+    boolean overrideAll(T t, V from, V to, Comparator<V> cmp);
+
+    T insert(T t, V v, Comparator<V> cmp);
+
+    boolean remove(T t, V v, Comparator<V> cmp);
+
+    boolean removeAll(T t, V v, Comparator<V> cmp);
+
+    V max(T t, Comparator<V> cmp);
+
+    V secondMax(T t, Comparator<V> cmp);
+
+    int height(T t);
+
+    boolean isBalanced(T t);
+
+    int numberOfNodes(T t);
+
+    int numberOfNodesOnLevel(T t, int level);
+
+    T rightmostNodeInLeftSubtree(T t);
+
+    T leftmostNodeInRightSubtree(T t);
+
+    T invert(T t);
+
+    T clone(T t);
+
+    /**
+     * Check whether ot not the given tree is "valid" according to
+     * what is considered valid depends on the specific data structure.
+     * This is also used by the tests to verify implementations.
+     * This has to be tested separate for each data structure.
+     *
+     * @param t A tree
+     * @return true iff the given tree is valid
+     */
+    boolean check(T t, Comparator<V> cmp);
+
+    /**
+     * Create an empty tree.
+     *
+     * @return A new, empty tree
+     */
+    T newEmptyTree();
+
+    /**
+     * Return an {@link Iterable}, or a lambda returning {@link java.util.Iterator},
+     * over the elements of the tree <code>t</code>.
+     * This is required by the tests.
+     *
+     * @param t A tree
+     * @return An {@link Iterable} over t
+     */
+    Iterable<V> iterate(T t);
+
+    /**
+     * Print a tree to standard out
+     *
+     * @param t A tree
+     */
+    void print(T t);
+}
\ No newline at end of file
diff --git a/src/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessor.java b/src/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessor.java
new file mode 100644
index 0000000..4fe012f
--- /dev/null
+++ b/src/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessor.java
@@ -0,0 +1,131 @@
+package aud.exam.prep.tree;
+
+import java.util.Comparator;
+
+public class RecursiveOrderedBinaryTreeNodeProcessor<V> extends OrderedBinaryTreeNodeProcessor<V> {
+
+    @Override
+    public boolean find(BinaryTreeNode<V> tree, V v, Comparator<V> cmp) {
+        return false;
+    }
+
+    @Override
+    public boolean override(BinaryTreeNode<V> pointer, V from, V to, Comparator<V> cmp) {
+        return false;
+    }
+
+    @Override
+    public boolean overrideAll(BinaryTreeNode<V> pointer, V from, V to, Comparator<V> cmp) {
+        return false;
+    }
+
+    @Override
+    public BinaryTreeNode<V> insert(BinaryTreeNode<V> tree, V v, Comparator<V> cmp) {
+        if (tree == null) {
+            tree = new BinaryTreeNode<>();
+            tree.key = v;
+            return tree;
+        }
+
+        var c = cmp.compare(v, tree.key);
+
+        if (c <= 0) {
+            tree.left = insert(tree.left, v, cmp);
+        } else {
+            tree.right = insert(tree.right, v, cmp);
+        }
+
+        return tree;
+    }
+
+    @Override
+    public boolean remove(BinaryTreeNode<V> tree, V v, Comparator<V> cmp) {
+        return false;
+    }
+
+    @Override
+    public boolean removeAll(BinaryTreeNode<V> tree, V v, Comparator<V> cmp) {
+        return false;
+    }
+
+    @Override
+    public V max(BinaryTreeNode<V> tree, Comparator<V> cmp) {
+        return null;
+    }
+
+    @Override
+    public V secondMax(BinaryTreeNode<V> tree, Comparator<V> cmp) {
+        return null;
+    }
+
+    @Override
+    public int height(BinaryTreeNode<V> tree) {
+        if (tree == null) {
+            return -1;
+        }
+        return 0;
+    }
+
+    @Override
+    public boolean isBalanced(BinaryTreeNode<V> tree) {
+        if (tree == null) {
+            return true;
+        }
+        return false;
+    }
+
+    @Override
+    public int numberOfNodes(BinaryTreeNode<V> tree) {
+        return 0;
+    }
+
+    @Override
+    public int numberOfNodesOnLevel(BinaryTreeNode<V> tree, int level) {
+        return 0;
+    }
+
+    @Override
+    public BinaryTreeNode<V> rightmostNodeInLeftSubtree(BinaryTreeNode<V> tree) {
+        return null;
+    }
+
+    @Override
+    public BinaryTreeNode<V> leftmostNodeInRightSubtree(BinaryTreeNode<V> tree) {
+        return null;
+    }
+
+    @Override
+    public BinaryTreeNode<V> invert(BinaryTreeNode<V> tree) {
+        return null;
+    }
+
+    @Override
+    public BinaryTreeNode<V> clone(BinaryTreeNode<V> tree) {
+        return null;
+    }
+
+    @Override
+    public boolean check(BinaryTreeNode<V> tree, Comparator<V> cmp) {
+        if (tree == null) {
+            return true;
+        }
+
+        if (tree.key == null) {
+            return false;
+        }
+
+        if (tree.left != null) {
+            if (!check(tree.left, cmp) || cmp.compare(tree.left.key, tree.key) > 0) {
+                return false;
+            }
+        }
+
+        if (tree.right != null) {
+            if (!check(tree.right, cmp) || cmp.compare(tree.right.key, tree.key) < 0) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+}
diff --git a/src/aud/exam/prep/tree/Stack.java b/src/aud/exam/prep/tree/Stack.java
new file mode 100644
index 0000000..81c1408
--- /dev/null
+++ b/src/aud/exam/prep/tree/Stack.java
@@ -0,0 +1,36 @@
+package aud.exam.prep.tree;
+
+import aud.exam.prep.array.Arrays;
+
+@SuppressWarnings("ManualArrayCopy")
+public class Stack<T> {
+
+    private int size = 0;
+    private T[] theStack = Arrays.newArray(10);
+
+    public void push(T t) {
+        while (size >= theStack.length) {
+            T[] newStack = Arrays.newArray(theStack.length * 2);
+
+            for (int i = 0; i < theStack.length; i++) {
+                newStack[i] = theStack[i];
+            }
+
+            theStack = newStack;
+        }
+
+        theStack[size++] = t;
+    }
+
+    public T peek() {
+        return theStack[size-1];
+    }
+
+    public T pop() {
+        return theStack[--size];
+    }
+
+    public boolean empty() {
+        return size == 0;
+    }
+}
diff --git a/test/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessorTest.java b/test/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessorTest.java
new file mode 100644
index 0000000..360c3ae
--- /dev/null
+++ b/test/aud/exam/prep/tree/OrderedBinaryTreeNodeProcessorTest.java
@@ -0,0 +1,96 @@
+package aud.exam.prep.tree;
+
+import org.junit.jupiter.api.Test;
+
+import static aud.exam.prep.Tests.CMP;
+import static org.junit.jupiter.api.Assertions.*;
+
+public abstract class OrderedBinaryTreeNodeProcessorTest extends OrderedTreeProcessorTest<BinaryTreeNode<Integer>> {
+
+    protected OrderedBinaryTreeNodeProcessorTest(OrderedTreeProcessor<Integer, BinaryTreeNode<Integer>> processor) {
+        super(processor);
+    }
+
+    @Test
+    void testThat_checkOfNull() {
+        assertTrue(processor.check(null, CMP));
+    }
+
+    @Test
+    void testThat_checkOfNoChildrenIsTrue() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 2;
+        assertTrue(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfNoKeyIsFalse() {
+        var t = new BinaryTreeNode<Integer>();
+        assertFalse(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfLeftGreaterIsFalse() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.left = new BinaryTreeNode<>();
+        t.left.key = 10;
+
+        assertFalse(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfLeftSmallerIsTrue() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.left = new BinaryTreeNode<>();
+        t.left.key = 2;
+
+        assertTrue(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfRightGreaterIsTrue() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.right = new BinaryTreeNode<>();
+        t.right.key = 10;
+
+        assertTrue(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfRightSmallerIsFalse() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.right = new BinaryTreeNode<>();
+        t.right.key = 2;
+
+        assertFalse(processor.check(t, CMP));
+    }
+
+    @Test
+    void testThat_checkOfBadRightIsFalse() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.right = new BinaryTreeNode<>();
+
+        assertFalse(processor.check(t, CMP));
+    }
+
+
+    @Test
+    void testThat_checkOfLeftIsFalse() {
+        var t = new BinaryTreeNode<Integer>();
+        t.key = 5;
+
+        t.left = new BinaryTreeNode<>();
+
+        assertFalse(processor.check(t, CMP));
+    }
+}
\ No newline at end of file
diff --git a/test/aud/exam/prep/tree/OrderedTreeProcessorTest.java b/test/aud/exam/prep/tree/OrderedTreeProcessorTest.java
new file mode 100644
index 0000000..5504803
--- /dev/null
+++ b/test/aud/exam/prep/tree/OrderedTreeProcessorTest.java
@@ -0,0 +1,55 @@
+package aud.exam.prep.tree;
+
+import aud.exam.prep.ListProvider;
+import aud.exam.prep.tree.OrderedTreeProcessor;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ArgumentsSource;
+
+import java.util.List;
+
+import static aud.exam.prep.Tests.CMP;
+import static org.junit.jupiter.api.Assertions.*;
+
+public abstract class OrderedTreeProcessorTest<T> {
+
+    protected final OrderedTreeProcessor<Integer, T> processor;
+
+    protected OrderedTreeProcessorTest(OrderedTreeProcessor<Integer, T> processor) {
+        this.processor = processor;
+    }
+
+    @Test
+    void testThat_newEmptyTreeWorks() {
+        T t = processor.newEmptyTree();
+
+        assertTrue(processor.check(t, CMP));
+
+        assertEquals(-1, processor.height(t));
+
+        assertTrue(processor.isBalanced(t));
+
+        assertEquals(0, processor.numberOfNodes(t));
+
+        for (int i = 0; i < 10; i++) {
+            assertEquals(0, processor.numberOfNodesOnLevel(t, i));
+        }
+
+        assertIterableEquals(List.of(), processor.iterate(t));
+    }
+
+
+    @ParameterizedTest
+    @ArgumentsSource(ListProvider.class)
+    void testThat_insertAndIterateWork(List<Integer> list) {
+        T t = processor.newEmptyTree();
+
+        for (var n : list) {
+            t = processor.insert(t, n, CMP);
+        }
+        list.sort(CMP);
+
+        assertTrue(processor.check(t, CMP));
+        assertIterableEquals(list, processor.iterate(t));
+    }
+}
\ No newline at end of file
diff --git a/test/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessorTest.java b/test/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessorTest.java
new file mode 100644
index 0000000..4584861
--- /dev/null
+++ b/test/aud/exam/prep/tree/RecursiveOrderedBinaryTreeNodeProcessorTest.java
@@ -0,0 +1,8 @@
+package aud.exam.prep.tree;
+
+public class RecursiveOrderedBinaryTreeNodeProcessorTest extends OrderedBinaryTreeNodeProcessorTest {
+
+    public RecursiveOrderedBinaryTreeNodeProcessorTest() {
+        super(new RecursiveOrderedBinaryTreeNodeProcessor<>());
+    }
+}
\ No newline at end of file