递归--逆波兰表达式

"""
第一种解法，list列表从后往前读入数据
当数据是数字则加入栈，如果是运算符，则取出栈顶的前2个数字进行计算
然后再把计算结果存入栈顶，
后续依次规则进行，最后求出结果

def Exp(list):
    stackList = []
    for i in reversed(list):
        if (i == "+"):
           num = len(stackList)
           newValue = float(stackList[num-1]) + float(stackList[num-2])
           stackList.pop()
           stackList.pop()
           stackList.append(newValue)
        elif (i == "-"):
            num = len(stackList)
            newValue = float(stackList[num - 2]) - float(stackList[num - 1])
            stackList.pop()
            stackList.pop()
            stackList.append(newValue)
        elif (i == "*"):
            num = len(stackList)
            newValue = float(stackList[num-1]) * float(stackList[num-2])
            stackList.pop()
            stackList.pop()
            stackList.append(newValue)
        elif (i == "/"):
            num = len(stackList)
            newValue = float(stackList[num - 2]) / float(stackList[num - 1])
            del stackList[-1]
            del stackList[-1]
            stackList.append(newValue)
        else:
            stackList.append(i)
    print(stackList[0])


def main():
    print("请输入需要求解的波兰表达式（前缀）：",end="")
    calculateList = []
    calculateList = input().strip().split(" ")
    Exp(calculateList)
"""
#第二种解法：利用函数递归调用实现

calculateList = []
#逐位遍历list列表，每次读入1个字符
pos = -1
def Exp():
    global pos
    pos = pos +1
    chr = calculateList[pos]
    if (chr == "+"):
        return Exp() + Exp()
    elif (chr == "-"):
        return Exp() - Exp()
    elif (chr == "*"):
        return Exp() * Exp()
    elif (chr == "/"):
        return Exp() / Exp()
    else:
        return float(chr)
def main():
    print("请输入需要求解的波兰表达式（前缀）：", end="")
    global calculateList
    calculateList = input().strip().split(" ")
    result = Exp()
    print("计算结果为：%f"%result)

if __name__ == "__main__":
    main()