说明：  对于MATLAB中的效率,计算单位向量之间的点乘比欧式距离更快捷。请注意: 注意角的比率(单位矢量点积的反余弦)是小角度的欧氏距离之比的近似值
(For efficiency in MATLAB, calculating dot multiplication between unit vectors is faster than Euclidean distance. Please note thatThe ratio of attention to angle (the cosine of dot product of unit vector) is the approximate value of the ratio of Euclidean distance of small angle.)