kd_root = kdtree_build( feat2, n2 );。
/** a node in a k-d tree */
struct kd_node{
int ki; /**< partition key index */
double kv; /**< partition key value */
int leaf; /**< 1 if node is a leaf, 0 otherwise */
struct feature* features; /**< features at this node */
int n; /**< number of features */
struct kd_node* kd_left; /**< left child */
struct kd_node* kd_right; /**< right child */
};