Evaluate reverse polish notation

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []

        for i in tokens:
            if i not in ['+', '-', '*', '/']:
                stack.append(int(i))
            else:
                num2 = stack.pop()
                num1 = stack.pop()
                print(num1, i ,num2)
                if i == '+':
                    stack.append(num1 + num2)
                elif i == '-':
                    stack.append(num1 - num2)
                elif i == '*':
                    stack.append(num1 * num2)
                elif i == '/':
                    stack.append(int(num1 / num2))
            # print(stack)
        return stack.pop()

Evaluate Reverse Polish Notation

Difficulty: Medium

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

 

Constraints:

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