用python实现中缀转后缀，并计算

import re
def convert(s):
    level = {}
    level['('] = 0
    level['+'] = 1
    level['-'] = 1
    level['*'] = 2
    level['/'] = 2
    output = []
    opStack = []
    list = s.split()
    for i in list:
        if re.match(r'^\d+$',i):
            output.append(i)
        elif i == '(':
            opStack.append(i)
        elif i == ')':
            top = opStack.pop()
            while top != '(':
                output.append(top)
                top = opStack.pop()
        elif i in '+-*/':
            while opStack and \
                    (opStack[-1] >= i):
                output.append(opStack.pop())
            opStack.append(i)
        else:
            raise Exception('meet a unexpected operator "%s"' % (i,))

    while opStack:
        output.append(opStack.pop())

    return output

def cal(a,b,op):
    if op=='+':
        return a+b
    elif op=='-':
        return a-b
    elif op=='*':
        return a*b
    elif op=='/':
        return a/b
    else:
        raise Exception('meet a unexpected operator "%s" ' % (op,))

def numEval(list):
    op = []
    for i in list:
        if re.match(r'^\d+$',i):
            op.append(int(i))
        elif i in '/*-+':
            a = op.pop()
            b = op.pop()
            op.append(cal(a,b,i))
        else:
            raise Exception('meet a unexpected operator "%s" ' % i)
    return op[0]



a = input('请输入中缀表达式')
after = convert(a)
print('后缀表达式为:' + ' '.join(after))
print('结果为:' + str(numEval(after)))

参考： 中缀前缀和后缀表达式