Undo Stack

Lab Java Advanced 30 points

Build a text editor with commands type WORD (append the word plus a space), undo (undo the last type, if any) and print (print the current text trimmed, or (empty)). Use a stack of previous states.

Input

n, then n commands.

Output

One line per print.

Example

Input

7
type hello
type big
print
undo
print
undo
print

Output

hello big
hello
(empty)

Sample input available Sample output available

Editor

Java

Hints

Before each change, push the old text; undo pops it back.

Solution explanation

Stacks naturally model undo: the most recent change is undone first.

Show solution

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        Deque<String> history = new ArrayDeque<String>();
        String text = "";
        for (int i = 0; i < n; i++) {
            String cmd = sc.next();
            if (cmd.equals("type")) {
                history.push(text);
                text = text + sc.next() + " ";
            } else if (cmd.equals("undo")) {
                if (!history.isEmpty()) {
                    text = history.pop();
                }
            } else {
                String shown = text.trim();
                System.out.println(shown.isEmpty() ? "(empty)" : shown);
            }
        }
    }
}

Output

Answer