Leetcode 150. Evaluate Reverse Polish Notation

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

It’s easy. Pushing numbers to stack and when there is an operator, take last two number from the stack and calculate, then put back to the stack.

The key thing is how to detect whether or not the upcoming token is a number.

isnumeric() and isdigit() doesn’t work for negative number.

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack = []

        for token in tokens:
            if bool(re.search(r'\d', token)):
                stack.append(int(token))
            else:
                num1 = stack.pop()
                num2 = stack.pop()
                if token == "+":
                    stack.append(num1 + num2)
                elif token == "-":
                    stack.append(num2 - num1)
                elif token == "*":
                    stack.append(num1 * num2)
                elif token == "/":
                    stack.append(int(num2 / num1))

        return stack[0]