Add memento pattern example for solution

Author: leemhoon00

src/Memento/Book/index.ts

@@ -0,0 +1,66 @@
+export interface Memento {
+  getName(): string;
+  getSnapshotDate(): Date;
+  getText(): string;
+}
+
+class Editor {
+  private text: string;
+
+  constructor(private name: string) {
+    this.text = "";
+  }
+
+  makeSnapshot(): Memento {
+    return new Snapshot(this.name, this.text);
+  }
+
+  restore(memento: Memento) {
+    this.text = memento.getText();
+  }
+
+  editText(newText: string) {
+    this.text = newText;
+  }
+
+  displayText() {
+    console.log("Current Text: " + this.text);
+  }
+}
+
+class Snapshot implements Memento {
+  private name: string;
+  private text: string;
+  private date: Date;
+
+  constructor(name: string, text: string) {
+    this.name = name;
+    this.text = text;
+    this.date = new Date();
+  }
+
+  getName(): string {
+    return this.name;
+  }
+
+  getSnapshotDate(): Date {
+    return this.date;
+  }
+
+  getText(): string {
+    return this.text;
+  }
+}
+
+const editor = new Editor("Document 1");
+
+editor.editText("This is the initial text");
+editor.displayText();
+
+const snapshot1 = editor.makeSnapshot();
+
+editor.editText("Text after editing");
+editor.displayText();
+
+editor.restore(snapshot1);
+editor.displayText();