menu = {
    "山东": {
        "济南": {
              "平阴": {},
              "商河": {},
              "莱芜": {}
         },
        "济宁": {
              "金乡": {},
              "微山": {},
              "嘉祥": {}
        },
        "曲阜": {
              "南辛": {},
              "陵城": {},
              "防山": {}
        }
    },
    "北京": {
        "海淀": {
                "万寿路": {},
                "永定路": {},
                "中关村": {}
       },
        "昌平": {
                "回龙观": {},
                "南口": {},
                "沙河": {
                    "路飞":{},
                    "娜美":{}
                }
        },
        "朝阳": {
                "呼家楼": {},
                "三里屯": {},
                "双井": {}
        }
    },
    "上海": {
        "黄浦": {
                "南京东路": {},
                "外滩": {},
                "瑞金二路": {}
        },
        "徐汇": {
            "湖南路": {},
            "天平路": {},
            "枫林路": {}

        },
        "长宁": {
                "华阳路": {},
                "新华路": {},
                "江苏路": {}
        }
    },
}
def func(m):
    current_layer = m
    layers = []
    while True:
        for k in current_layer: #menu[北京]
            print(k)
        choice = input(">>:").strip()
        if not choice:continue
        if choice in current_layer:
            layers.append(current_layer)#进入下一层，保存当前层
            current_layer = current_layer[choice] #menu[北京][昌平]
        elif choice == 'b':
            if len(layers) != 0:
                current_layer= layers.pop()
            else:
                print("已经在最顶层了")
        elif choice == 'q':
            print('bye......')
            break
func(menu)