150.py

'''
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].
'''

# https://leetcode.com/problems/evaluate-reverse-polish-notation/

import unittest

def evalRPN(tokens):
    stack = []
    for token in tokens:
        if token in '+-*/':
            b = stack.pop()
            a = stack.pop()
            if token == '+':
                stack.append(a + b)
            elif token == '-':
                stack.append(a - b)
            elif token == '*':
                stack.append(a * b)
            else:
                stack.append(int(a / b))
        else:
            stack.append(int(token))
    return stack[0]

class TestEvalRPN(unittest.TestCase):
    def test1(self):
        self.assertEqual(evalRPN(["2","1","+","3","*"]), 9)

    def test2(self):
        self.assertEqual(evalRPN(["4","13","5","/","+"]), 6)

    def test3(self):
        self.assertEqual(evalRPN(["10","6","9","3","+","-11","*","/","*","17","+","5","+"]), 22)

if __name__ == '__main__':
    unittest.main()