def parse(self, exp):
        opers = []
        output = []
        for x in exp:
            if x in self.ops:
                if opers:
                    while opers and self.ops.get(opers[-1]) >= self.ops.get(x):
                        output.append(opers.pop())
                opers.append(x)
            elif x == '(':
                opers.append(x)
            elif x == ')':
                while opers and opers[-1] != '(':
                    output.append(opers.pop())
                opers.pop()
            else:
                output.append(x)
        while opers:
            x = opers.pop()
            if x != '(' and x != ')':
                output.append(x)
        return output