说明：  k-d树（k-dimensional树的简称），是一种分割k维数据空间的数据结构。主要应用于多维空间关键数据的搜索（如：范围搜索和最近邻搜索）。K-D树是二进制空间分割树的特殊的情况。
(kd tree (k-dimensional tree for short), is a data structure split k-dimensional data space. Mainly used in multi-dimensional space-critical data search (eg: range search and nearest neighbor search). KD tree is a special case of binary space partitioning tree.)