0150-evaluate-reverse-polish-notation

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Description

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].

Solution(Python)

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        #  tokens = ["2","1","+","3","*"]
        # i =0 2
        # stk = [2]
        # i =1 1 
        #  stk =[2 1]
        # i =2  "+"
        #   pop two times num2 num1  1 2 do op 3
        #   add 3 to stck
        # stk =3
        # stk 33 
        #  * 
        def doOp(op, a, b):
            match(op):
                case "+":
                    return a + b
                case "-":
                    return a - b
                case "*":
                    return a * b
                case "/":
                    return int(a/b)
        
        stk = []
        out = 0

        for token in tokens:
            if token in "+-*/":
                num2 = stk.pop()
                num1 = stk.pop()
                res = doOp(token, num1, num2)
                stk.append(res)
            else:
                stk.append(int(token))
        return stk.pop()

Dry Run