三级菜单，可以退出到上一级菜单和全部退出（low版本）

menu = {    ‘北京‘:{        ‘海淀‘:{            ‘五道口‘:{                ‘soho‘:{},                ‘网易‘:{},                ‘google‘:{}            },            ‘中关村‘:{                ‘爱奇艺‘:{},                ‘汽车之家‘:{},                ‘youku‘:{},            },            ‘上地‘:{                ‘百度‘:{},            },        },        ‘昌平‘:{            ‘沙河‘:{                ‘老男孩‘:{},                ‘北航‘:{},            },            ‘天通苑‘:{},            ‘回龙观‘:{},        },        ‘朝阳‘:{},        ‘东城‘:{},    },    ‘上海‘:{        ‘闵行‘:{            "人民广场":{                ‘炸鸡店‘:{}            }        },        ‘闸北‘:{            ‘火车战‘:{                ‘携程‘:{}            }        },        ‘浦东‘:{},    },    ‘山东‘:{},}

while True:    for key in menu:        print(key)    choice = input("请输入>>:").strip()    if len(choice) == 0:continue    if choice == "b":break    if choice == "q":exit()    if choice not in menu:continue

while True:        for key2 in menu[choice]:            print(key2)        choice2 = input("请输入>>:").strip()        if len(choice2) == 0: continue        if choice2 == "b": break        if choice2 == "q": exit()        if choice2 not in menu[choice]: continue

while True:            for key3 in menu[choice][choice2]:                print(key3)            choice3 = input("请输入>>:").strip()            if len(choice3) == 0: continue            if choice3 == "b": break            if choice3 == "q": exit()            if choice3 not in menu[choice][choice2]: continue

while True:                for key4 in menu[choice][choice2][choice3]:                    print(key4)                choice4 = input("请输入>>:").strip()                if len(choice4) == 0: continue                if choice4 == "b": break                if choice4 == "q": exit()                if choice4 not in menu[choice][choice2][choice3]: continue