简易计算器 输入数学表达式，输出表达式的计算结果。数学表达式由单个数字和运 算符“ +”、“ -”、“ *”、“ /”、“ (、“ )构成，例如 2 + 3 * ( 4 + 5 ) – 6 / 4。假定 表达式输入格式合法。-Enter the mathematical expression, calculation result output expression. Mathematical expressions and shipped by a single digit O