150. Evaluate Reverse Polish Notation.py

'''
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, arr: List[str]) -> int:
        stack = []
        for i in range(len(arr)):
            if arr[i]=='+':
                num1 = stack.pop()
                num2 = stack.pop()
                stack.append(num2+num1)
            elif arr[i]=='-':
                num1 = stack.pop()
                num2 = stack.pop()
                stack.append(num2-num1)
            elif arr[i]=='*':
                num1 = stack.pop()
                num2 = stack.pop()
                stack.append(num2*num1)
            elif arr[i]=='/':
                num1 = stack.pop()
                num2 = stack.pop()
                stack.append(int(num2/num1))
            else:
                stack.append(int(arr[i]))
            # print(stack)
        return stack[0]