Evaluate Reverse Polish Notation — Python

# Time:  O(n)
# Space: O(n)

import operator

class Solution(object):
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        numerals, operators = [], {"+": operator.add, "-": operator.sub, "*": operator.mul, "/": operator.div}
        for token in tokens:
            if token not in operators:
                numerals.append(int(token))
            else:
                y, x = numerals.pop(), numerals.pop()
                numerals.append(int(operators[token](x * 1.0, y)))
        return numerals.pop()