"""
150. Evaluate Reverse Polish Notation
Difficulty: Medium
https://leetcode.com/problems/evaluate-reverse-polish-notation/

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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 <= 10^4

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


class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []
        operations = {
            "+": lambda x, y: int(x + y),
            "-": lambda x, y: int(x - y),
            "*": lambda x, y: int(x * y),
            "/": lambda x, y: int(x / y),
        }

        for c in tokens:
            if c in operations:
                y = stack.pop()
                x = stack.pop()
                calc_func = operations[c]
                result = calc_func(x, y)
                stack.append(result)
            else:
                stack.append(int(c))

        return stack[0]