为什么PTA中答案和格式都对但是提交时还是报格式错误

class Stack:
    def __init__(self):
        self.items = []
    def isEmpty(self):
        return self.items == []
    def top(self):
        if self.items == []:
            raise StackUnderflow
        return self.items[len(self.items) - 1]
    def push(self,item):
        return self.items.append(item)
    def pop(self):
        return self.items.pop()
    def size(self):
        return len(self.items)

operators = {"(": 1, "+": 3, "-": 3, "*": 5, "/": 5}
infix_operators = "+-*/()"

def houzhui(k):
    s = Stack()     #放操作符
    exp = []        #放数字
    a = despace(k)
    for i in range(len(a)):
        if a[i] == "(":  # 判别“（”
            s.push(a[i])
        elif a[i] not in infix_operators:
            exp.append(a[i])
        elif a[i] == ")":
            while not s.isEmpty() and s.top() != "(":
                exp.append(s.pop())
            if s.isEmpty():
                raise SyntaxError("Missing \'(\'.")
            s.pop()    #删除栈中的“(”

        else:
            while(not s.isEmpty() and operators[a[i]] <= operators[s.top()]):
                exp.append(s.pop())
            s.push(a[i])
    while not s.isEmpty():
        if s.top() == "(":
            raise SyntaxError("Extra\'(\' in expression.")
        exp.append(s.pop())
    return exp

def despace(s):
    arr = []
    for i in range(len(s)):
        if s[i].isspace():
            continue
        arr.append(s[i])
    return arr

a = input()
b = houzhui(a)
print(" ".join(b))