Postfix Notation
后缀表达式（postfix notation，也叫逆波兰表达式，reverse polish notation，RPN） 是运算表达式的一种形式，运算符在操作数之后，比如 2 2 +。
常用于计算机处理，特点是没有括号，所有计算按运算符出现顺序从左到右进行。
Wikipedia
Reverse Polish notation (RPN), also known as reverse Łukasiewicz notation, Polish postfix notation or simply postfix notation, is a mathematical notation in which operators follow their operands, in contrast to Polish notation (PN), in which operators precede their operands. It does not need any parentheses as long as each operator has a fixed number of operands.[1]