leetcode 150. 逆波兰表达式求值

 150. 逆波兰表达式求值 - 力扣（LeetCode）

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack=[]
        for item in tokens:
            if item not in ( '+','-','*' , '/' ):
                stack.append(item)
            else:
                pre=int(stack.pop())
                pre_before=int(stack.pop())
                sign =item
                if sign =='/':
                    result=int(pre_before/pre)
                elif sign =='+':
                    result=pre_before+pre
                elif sign =='-':
                    result=pre_before-pre
                elif sign =='*':
                    result=pre_before*pre
                stack.append(result)
        return int(stack.pop())