150 逆波兰表达式

class Solution:
    def evalRPN(self, tokens: List[str]) -> int:
        stack=[]
        res=0
        for i in tokens:
            if i.isdigit():
                stack.append(i)
            else:
                if i=='/':
                    i='//'
                b=stack.pop()
                a=stack.pop()
                ans=eval(a+i+b)
                stack.append(str(ans))
        return stack[0]