计算逆波兰式(后缀表达式)的值
运算符仅包含"+","-","*"和"/",被操作数是整数
保证表达式合法,除法时向下取整。
数据范围:表达式的长度满足: 
进阶:空间复杂度
, 时间复杂度 )
进阶:空间复杂度
例如:
["20", "10", "+", "30", "*"] -> ((20 + 10) * 30) -> 900 ["40", "130", "50", "/", "+"] -> (40 + (130 / 50)) -> 42
["20", "10", "+", "30", "*"] -> ((20 + 10) * 30) -> 900 ["40", "130", "50", "/", "+"] -> (40 + (130 / 50)) -> 42
["20","10","+","30","*"]
900
["20"]
20