先实现了将前缀表达式转换为逆波兰表达式的功能, 之后再利用逆波兰表达式对表达式进行求值, 支持+,-,*,/,^等运算, 以及sqrt, lg, ln, exp, sin, cos, tan, asin, acos, atan, sh, ch等函数操作-First realized the prefix expressions into reverse Polish notation features, re-use reverse Polish expression after the exp