后缀式转中缀式 将由数字和四则运算符组成的后缀表达式变换为中缀表达式。 输入的后缀表达式包含的运算符不超过15个。 要求转换后的中缀表达式中不应出现不必要的括号。例如， 整个表达式两端的括号要省略，不影响原计算顺序的括号要省 略。-Suffix-type switch-type will be made up of four numbers and the composition operator for suffix expressions transform inf

简单的赋值语句转化为四元式 只对单字符赋值 以及表达式的四元式转换-Simple assignment statements into four type Only to the single character assignment as well as the expression of quaternion type conversion