150. Evaluate Reverse Polish Notation【M】【19】

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

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class Solution(object):
    def evalRPN(self, tokens):

        s = []

        for i in tokens:

            if i == "+":
                t = s[-1] + s[-2]
                s = s[:-2]
                s += t,
            elif i == "-":
                t = s[-2] - s[-1]
                s = s[:-2]
                s += t,
            elif i == "*":
                t = s[-2] * s[-1]
                s = s[:-2]
                s += t,
            elif i == "/":
                if s[-2]*s[-1] < 0:
                    t = - (abs(s[-2]) / abs(s[-1]))
                else:
                    t = s[-2] / s[-1]
                s = s[:-2]
                s += t,
            else:
                i = int(i)
                s += i,
                #print s
        return s[0]