文本编辑器撤销

def solve():
    class TextEditor:
        def __init__(____):
            self._text = '____'
            self._history = []
        def write(____):
            self._history.append(____)
            self._text += text
        def undo(____):
            if self._history:
                self._text = self._history.pop()
    editor = TextEditor()
    editor.write(____)
    editor.write(____)
    editor.undo()
    print(____)