兩種查找算法的比較

一、普通查找：雙層循環遍歷，第二層循環中找到即break，查找時間複雜度O(M*N/2)

List<PtCameraInfo> cameraList = new List<PtCameraInfo>();
List<string> cameraIdList = dataIds.Split(',').ToList();
List<PtCameraInfo> oldList = this.cameraList.Cameras.ToList();

for (int i = 0; i < cameraIdList.Count; i++)
{
    string cameraId = cameraIdList[i];
    for (int j = 0; j < _cameraList.Count; j++)
    {
        PtCameraInfo camera = _cameraList[j];
        if (camera.ID == cameraId && !oldList.Exists(a => a.ID == camera.ID))
        {
            cameraList.Add(camera);
            break;
        }
    }
}

View Code

二、高效查找：排序雖然費時，但數量級低，因此耗時很低，排序時間複雜度O(M*logM+N*logN)，查找過程也是雙層循環，但第二層循環比較省時，查找時間複雜度O(M*logN)，總的時間複雜度：O(M*logM+N*logN+M*logN)

List<PtCameraInfo> cameraList = new List<PtCameraInfo>();
List<string> cameraIdList = dataIds.Split(',').ToList();
List<PtCameraInfo> oldList = this.cameraList.Cameras.ToList();

//排序
this._cameraList.Sort(new Comparison<PtCameraInfo>((a, b) =>
{
    return string.Compare(a.ID.PadLeft(36, '0'), b.ID.PadLeft(36, '0'));
}));
cameraIdList.Sort((a, b) =>
{
    return string.Compare(a.PadLeft(36, '0'), b.PadLeft(36, '0'));
});

//高效查找
int k = 0;
for (int i = 0; i < cameraIdList.Count; i++)
{
    string cameraId = cameraIdList[i];
    for (int j = k; j < _cameraList.Count; j++)
    {
        PtCameraInfo camera = _cameraList[j];
        if (camera.ID == cameraId && !oldList.Exists(a => a.ID == camera.ID))
        {
            cameraList.Add(camera);
            k = j;
            break;
        }
    }
}

View Code

說明：cameraList的數量級是2萬，當cameraIdList數量從1到大約1000時，高效查找耗時始終是0.1秒多左右，而普通查找，當cameraIdList數量不多時，很快，0.05秒，當cameraIdList數量接近1000時，大約2秒。