二分查找法查找城市名称

class City
{
    public string CityName { get; set; }

    public int Start { get; set; }

    public int End { get; set; }

    public bool IsCurrentCity(int num)
    {
        return  (num >= Start && num <= End);
   }
}

class CityHelper
{
    List<City> cityList = null;

    public string GetCityName(int num)
    {
        cityList = GetCityList();

        string name = null;
        int start = 0;
        int end = cityList.Count - 1;

        while (end >= start)
        {
            int middle = (start + end) / 2;

            if (cityList[middle].IsCurrentCity(num))
            {
                name = cityList[middle].CityName;
                break;
            }

            if (cityList[middle].End < num)
                start = middle + 1;
            else if (cityList[middle].End > num)
                end = middle - 1;
        }

        return name;
    }

    List<City> GetCityList()
    {
        if (cityList == null)//判断为空再创建
        {
            cityList = new List<City>();

            City model = new City();
            model.CityName = "北京";
            model.Start = 1310000;
            model.End = 1310000 + 875;
            cityList.Add(model);

            model = new City();
            model.CityName = "天津";
            model.Start = 1311000;
            model.End = 1311000 + 275;
            cityList.Add(model);

            model = new City();
            model.CityName = "石家庄";
            model.Start = 1312000;
            model.End = 1312000 + 875;
            cityList.Add(model);

            model = new City();
            model.CityName = "济南";
            model.Start = 1313500;
            model.End = 1313500 + 75;
            cityList.Add(model);
        }
        return cityList;
    }
}