3.21 假设表达式有单字母变量和双目四则运算符构成。试写一个算法，将一个通常书写形式且书写正确的表达式转换为逆波兰表达式。-3.21 Suppose a single-letter variables and expressions binocular four operators composition. Try to write an algorithm that will form a typically write and write the correct expression is converted to Reverse Polish Notation.