Evaluate Reverse Polish Notation Leetcode Python

Evaluate the value of an arithmetic expression in Reverse Polish Notation.


Valid operators are +, -, *, /. Each operand may be an integer or another expression.


Some examples:
  ["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9

  ["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

We need to use an extra stack to solve this problem.

1.append all the none symbol values to the stack,

the pop two element everytime and then take sum/deduct/multiply/divide 

the code is as follow:

class Solution:
    # @param tokens, a list of string
    # @return an integer
    def evalRPN(self, tokens):
        stack=[]
        for index in range(len(tokens)):
            if tokens[index]!='+' and tokens[index]!='-' and tokens[index]!='*' and tokens[index]!='/':
                stack.append(int(tokens[index]))
            else:
                a=int(stack.pop())
                b=int(stack.pop())
                if tokens[index]=='+':
                    stack.append(a+b)
                if tokens[index]=='-':
                    stack.append(b-a)
                if tokens[index]=='*':
                    stack.append(a*b)
                if tokens[index]=='/':
                    if a*b<0:
                        stack.append(-(-b/a))
                    else:
                        stack.append(b/a)
        return stack.pop()