150. Evaluate Reverse Polish Notation

Evaluate the value of an arithmetic expression in Reverse Polish Notation.

Valid operators are +, -, *, /. Each operand may be an integer or another expression.

Some examples:

["2", "1", "+", "3", "*"] -> ((2 + 1) * 3) -> 9
["4", "13", "5", "/", "+"] -> (4 + (13 / 5)) -> 6

Solution: Stack

OP = {
    '+': lambda a, b: a + b,
    '-': lambda a, b: a - b,
    '*': lambda a, b: a * b,
    '/': lambda a, b: int(float(a) / b)
}


class Solution(object):
    def evalRPN(self, tokens):
        """
        :type tokens: List[str]
        :rtype: int
        """
        stack = []
        for token in tokens:
            try:
                num = int(token)
                stack.append(num)
            except ValueError:
                if token not in OP:
                    raise Exception("invalid operator")
                if len(stack) < 2:
                    raise Exception("invalid numbers")
                b = stack.pop()
                a = stack.pop()
                num = OP[token](a, b)
                stack.append(num)
        if len(stack) != 1:
            raise Exception("invalid numbers")
        return stack.pop()

Lessons:

• Python evaluates -1 / 2 into -1 instead of 0, so we use int(float(a) / b) for division.