【问题】

Evaluate the value of an arithmetic expression in .

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

【代码】

class Solution:    # @param tokens, a list of string    # @return an integer    def evalRPN(self, tokens):        stack = []        for item in tokens:            if item not in ("+", "-", "*", "/"):                stack.append(int(item))            else:                op2 = stack.pop()                op1 = stack.pop()                if item == "+":                    stack.append(op1 + op2)                elif item == "-":                    stack.append(op1 - op2)                elif item == "*":                    stack.append(op1 * op2)                elif item == "/":                    stack.append(int(op1 *1.0 / op2))        return stack[0]