Evaluate Reverse Polish Notation

erpn.py

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        """Calculates tokens and returns the result.
        
        Use stack to store numbers and pop two numbers once 
        meets operators then push the calculated result back.
        """
        OPS = {
                '+': lambda x, y: x+y,
                '-': lambda x, y: y-x,
                '*': lambda x, y: x*y,
                '/': lambda x, y: y/float(x)
            }
        
        stack = []
        for token in tokens:
            if token in OPS:
                stack.append(int(OPS[token](stack.pop(), stack.pop())))
            else:
                stack.append(int(token))
        
        return stack.pop()