Command pattern for undo in LLD - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the Command interface method for undo functionality.

LLD

interface Command {
    void execute();
    void [1]();
}

Drag options to blanks, or click blank then click option'

Aundo

Bredo

Crun

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'redo' instead of 'undo' as the method name.

Using unrelated method names like 'run' or 'start'.

The Command interface must declare an undo method to support undo operations.

2fill in blank

medium

Complete the code to add a command to the undo stack after execution.

LLD

public class Invoker {
    private Stack<Command> undoStack = new Stack<>();

    public void executeCommand(Command cmd) {
        cmd.execute();
        undoStack.[1](cmd);
    }
}

Drag options to blanks, or click blank then click option'

Apush

Bremove

Cpeek

Dpop

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'pop' which removes an element instead of adding.

Using 'peek' which only views the top element.

After executing a command, it should be pushed onto the undo stack to enable undoing later.

3fill in blank

hard

Fix the error in the undo method to correctly undo the last command.

LLD

public void undo() {
    if (!undoStack.isEmpty()) {
        Command cmd = undoStack.[1]();
        cmd.undo();
    }
}

Drag options to blanks, or click blank then click option'

Apeek

Bpop

Cpush

Dadd

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'peek' which does not remove the command from the stack.

Using 'push' which adds instead of removes.

To undo the last command, it must be popped from the undo stack before calling undo.

4fill in blank

hard

Fill both blanks to implement redo functionality using a redo stack.

LLD

public class Invoker {
    private Stack<Command> undoStack = new Stack<>();
    private Stack<Command> redoStack = new Stack<>();

    public void undo() {
        if (!undoStack.isEmpty()) {
            Command cmd = undoStack.[1]();
            cmd.undo();
            redoStack.[2](cmd);
        }
    }
}

Drag options to blanks, or click blank then click option'

Apop

Bpush

Cpeek

Dremove

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'peek' instead of 'pop' to remove from undoStack.

Using 'pop' instead of 'push' to add to redoStack.

Undo removes the last command from undoStack using pop, then adds it to redoStack using push for redo support.

5fill in blank

hard

Fill all three blanks to implement redo method that re-executes and updates stacks correctly.

LLD

public void redo() {
    if (!redoStack.isEmpty()) {
        Command cmd = redoStack.[1]();
        cmd.[2]();
        undoStack.[3](cmd);
    }
}

Drag options to blanks, or click blank then click option'

Apop

Bexecute

Cpush

Dundo

Attempts:

3 left

💡 Hint

Common Mistakes

Calling undo instead of execute during redo.

Using peek instead of pop to remove from redoStack.

Redo removes the last command from redoStack with pop, executes it with execute, then pushes it back to undoStack with push.