题解 | 牛牛与后缀表达式

#
# 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
#
# 给定一个后缀表达式，返回它的结果
# @param str string字符串
# @return long长整型
#
class Solution:
    def legalExp(self, st):
        stack = []
        s = ""
        num = [str(i) for i in range(10)]
        fuhao = list("+-*")
        for ch in st:
            if ch in num:
                s = str(s) + ch
            elif ch == "#":
                stack.append(int(s))
                s=""
            elif ch in fuhao:
                if ch == fuhao[0]:
                    a = stack.pop()
                    b = stack.pop()
                    stack.append(b + a)
                elif ch == fuhao[1]:
                    a = stack.pop()
                    b = stack.pop()
                    stack.append(b - a)
                elif ch == fuhao[2]:
                    a = stack.pop()
                    b = stack.pop()
                    stack.append(b * a)
        return stack[-1]