树状结构，无限极分类

def xtree(data):
    list = []
    dict = {}
    if data:
        for j in data:
            dict[j['id']] = j

        for i in data:
            if i['pid'] == 0:
                list.append(i)
            else:
                if 'son' not in dict[i['pid']]:
                    dict[i['pid']]['son'] = []
                dict[i['pid']]['son'].append(i)
        return list