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Evaluate the value of an arithmetic expression in .

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

Note:

• Division between two integers should truncate toward zero.
• The given RPN expression is always valid. That means the expression would always evaluate to a result and there won't be any divide by zero operation.

Example 1:

Input: ["2", "1", "+", "3", "*"]Output: 9Explanation: ((2 + 1) * 3) = 9

Example 2:

Input: ["4", "13", "5", "/", "+"]Output: 6Explanation: (4 + (13 / 5)) = 6

用一个stack来存储数字和符号。

遇到符号时，把前两个数字pop()出来进行计算。

python比较闹心的就是除法上，3//2=1， -3//2=-2。所以我直接都转为正数进行计算，再加入符号。

class Solution(object):    def evalRPN(self, tokens):        """        :type tokens: List[str]        :rtype: int        """        if len(tokens)==0:return        stack=[]                for c in tokens:            if c in '+-*/':                b=int(stack.pop())                a=int(stack.pop())                if c=='+':                    stack.append(a+b)                elif c=='-':                    stack.append(a-b)                elif c=='*':                    stack.append(a*b)                else:                    num=abs(a)//abs(b)                    sym=-1 if a*b<0 else 1                    stack.append(sym*num)            else:                stack.append(c)                return stack[0]

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