["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

 1 def is_op(c): 2     return c in [‘+‘, ‘-‘, ‘*‘, ‘/‘] 3  4 def divide(x, y): 5     if (x * y) < 0: 6         return -1 * ((-x)/y) 7     return x/y 8  9 class Solution:10     # @param tokens, a list of string11     # @return an integer12     def evalRPN(self, tokens):13         opDict = {‘+‘: lambda x,y: x+y,14                   ‘-‘: lambda x,y: x-y,15                   ‘*‘: lambda x,y: x*y,16                   ‘/‘: divide}17         record = []18 19         for item in tokens:20             if is_op(item):21                 second = record.pop()22                 first = record.pop()23                 record.append(opDict[item](first, second))24             else:25                 record.append(int(item))26         return record[0]