Add all 23 GoF design pattern implementations (2026-06-13)

03-behavioral/memento/History.java

@@ -0,0 +1,24 @@
+package memento;
+
+import java.util.ArrayDeque;
+import java.util.Deque;
+
+/**
+ * Caretaker — manages the stack of mementos.
+ * It does NOT open or inspect the mementos; it just stores and returns them.
+ */
+public class History {
+    private final Deque<EditorMemento> snapshots = new ArrayDeque<>();
+
+    public void save(EditorMemento memento) {
+        snapshots.push(memento);
+        System.out.println("  [History] Saved: " + memento);
+    }
+
+    public EditorMemento undo() {
+        if (snapshots.isEmpty()) return null;
+        return snapshots.pop();
+    }
+
+    public int size() { return snapshots.size(); }
+}