Evaluate Reverse Polish Notation

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

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

Note that division between two integers should truncate toward zero.

It is guaranteed that the given RPN expression is always valid. That means the expression would always evaluate to a result, and there will not be any division by zero operation.

Example 1:

Input: tokens = ["2","1","+","3","*"]
Output: 9
Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: tokens = ["4","13","5","/","+"]
Output: 6
Explanation: (4 + (13 / 5)) = 6

Example 3:

Input: tokens = ["10","6","9","3","+","-11","*","/","*","17","+","5","+"]
Output: 22
Explanation: ((10 * (6 / ((9 + 3) * -11))) + 17) + 5
= ((10 * (6 / (12 * -11))) + 17) + 5
= ((10 * (6 / -132)) + 17) + 5
= ((10 * 0) + 17) + 5
= (0 + 17) + 5
= 17 + 5
= 22

Constraints:

1 <= tokens.length <= 10⁴
tokens[i] is either an operator: "+", "-", "*", or "/", or an integer in the range [-200, 200].



class Solution:

    def  result(self, n1, n2, op):
        if op == '+':
            return n1+n2
        elif op == '-':
            return n1-n2
        elif op == '*':
            return n1*n2
        else:
            return int(n1 / n2)

    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        operators = ('+', '-', '*', '/')
        r = None

        for item in tokens:
            if item not in operators:
                stack.append(item)
            else:
                val2 = stack.pop()
                val1 = stack.pop()

                r = self.result(int(val1), int(val2), item)
                #print(r)
                #print(val1, item, val2, r)
                stack.append(r)

        return int(stack.pop())

Evaluate Reverse Polish Notation

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