Add an OrderedSet datatype

jlib/OrderedSet.java

@@ -0,0 +1,36 @@
+package jlib;
+
+import java.util.HashSet;
+import java.util.Stack;
+
+public class OrderedSet<T> extends HashSet<T> {
+	private Stack<T> order;
+
+	public OrderedSet() {
+		this.order = new Stack<T>();
+	}
+
+	@Override
+	public boolean add(T e) {
+		if (super.add(e)) {
+			this.order.add(e);
+			return true;
+		}
+		return false;
+	}
+
+	/**
+	 * Removes the most recently added element from the set
+	 *
+	 * @return The element that was removed from the set
+	 */
+	public T pop() {
+		T e = this.order.pop();
+		this.remove(e);
+		return e;
+	}
+
+	public T top() {
+		return this.order.peek();
+	}
+}