Evaluate Reverse Polish Notation

Problem Statement

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.

Leetcode Link

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].

Code

Python

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        def fun(st,n1,n2):
            s1,s2=1,1
            if n1<0:
                s1=-1
            if n2<0:
                s2=-1
            if st=='+':
                return n1+n2
            elif st=='-':
                return n1-n2
            elif st=='*':
                return n1*n2
            elif st=='/':
                return floor(abs(n1)/abs(n2))*(s1*s2)
        digits=[]
        for i in tokens:
            if i.isdigit() or (len(i)>1):
                digits.append(int(i))
            else:
                n1=digits.pop()
                n2=digits.pop()
                digits.append(fun(i,n2,n1))
        return digits[0]