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150. Evaluate Reverse Polish Notation

Difficulty: Medium

Related Topics: Array, Math, Stack

You are given an array of strings tokens that represents an arithmetic expression in a Reverse Polish Notation.

Evaluate the expression. Return an integer that represents the value of the expression.

Note that:

The valid operators are ‘+’, ‘-‘, ‘*’, and ‘/’. Each operand may be an integer or another expression. The division between two integers always truncates toward zero. There will not be any division by zero. The input represents a valid arithmetic expression in a reverse polish notation. The answer and all the intermediate calculations can be represented in a 32-bit integer.

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

Code:

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        if len(tokens) == 1: return int(tokens[0])
        operators = ["+", "-", "*", "/"]
        stack = []
        for token in tokens:
            if token in operators:
                sec = int(stack.pop())
                fir = int(stack.pop())
                # stack.append(int(eval(str(fir)+token+str(sec))))   # Takes twice of time to run
                if token == "+":
                    stack.append(fir+sec)
                elif token == "-":
                    stack.append(fir-sec)
                elif token == "*":
                    stack.append(fir * sec)
                else:
                    stack.append(int(fir/sec))
            else:
                stack.append(token)
        return stack[0]

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