Command Pattern Apply/Undo

Problem

Simulate a counter driven by reversible command objects.

operations contains "apply" (add values[i] to the counter and push the command), "undo" (revert the most recent applied command), "redo" (reapply the most recently undone command), or "value" (record the counter). A new apply clears the redo stack.

Return the list of recorded results (None for apply, undo, and redo).

Hint

Commands are deltas, so undo subtracts and redo adds the stored amount.

Test cases

case 1

in: [["apply","apply","undo","value","redo","value"],[3,4,0,0,0,0]]

expected: [null,null,null,3,null,7]

case 2

in: [["undo","value"],[0,0]]

expected: [null,0]

case 3

in: [["redo"],[0]]

expected: [null]

case 4

in: [["apply","undo","apply","redo","value"],[5,0,10,0,0]]

expected: [null,null,null,null,10]

Starter codePython

def command_ops(operations, values):
    # Your code here
    pass

4 test cases · run your code against them in the editor