功能强大的计算器，例如输入2*（-2+3*2）/4一个表达式后按下等号=，则显示 2*（-2+3*2）/4=2,有清零和后退一位改正输错的一位。能阻止错误输入行为。-Powerful calculator, such as type 2* (-2+3* 2)/4, press the equal sign after an expression =, it shows 2* (-2+3* 2)/4 = 2, there is zero and back, a correction of a