[LC] 150. Evaluate Reverse Polish Notation

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

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

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

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

Example 2:

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

Example 3:

Input: ["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

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        import string
        nums = string.digits
        operator = ['+', '-', '*', '/']

        for token in tokens:
            if token in operator:
                last_one = stack.pop()
                last_two = stack.pop()
                cur_val = self.helper(last_one, last_two, token)
                stack.append(cur_val)
            else:
                stack.append(int(token))
        return stack.pop()
    
    def helper(self, last_one, last_two, operator):
        if operator == '+':
            return last_two + last_one
        elif operator == '-':
            return last_two - last_one
        elif operator == '*':
            return last_two * last_one
        elif operator == '/':
            # special case to get ceiling value 
            if last_two * last_one < 0 and last_two % last_one != 0:
                return last_two // last_one + 1
            else:
                return last_two // last_one
        else:
            return