//KDTree::是由於定義了KDTree的namespace
KDTree::KDTreeNode* KDTree::KDTree::createKDTree( const ExamplarSet &exm_set )
{
    if(exm_set.empty())
        return NULL;

    ExamplarSet exm_set_copy(exm_set);

    int dims = exm_set_copy.getDims();
    int size = exm_set_copy.getSize();

    //計算每個維的方差，選出方差值最大的維    double
 
 var_max = -0.1; 
    double avg, var;
    int dim_max_var = -1;
    for(int i=0;i<dims;i++)
    {
        avg = 0;
        var = 0;
        //求某一維的總和        for(int j=0;j<size;j++)
        {
            avg += exm_set_copy[j][i];
        }
        //求平均        avg /= size;
        //求方差        for(int j=0;j<size;j++)
        {
            var
 
 += ( exm_set_copy[j][i] - avg ) * 
                ( exm_set_copy[j][i] - avg );
        }
        var /= size;
        if(var > var_max)
        {
            var_max = var;
            dim_max_var = i;
        }
    }

    //確定節點的資料向量    _HyperRectangle hr = exm_set_copy.calculateRange();     //統計節點空間範圍    exm_set_copy.sortByDim(dim_max_var);                    //
 
將所有資料向量按最大區分度方向排序    int mid = size / 2;
    _Examplar exm_split = exm_set_copy.examplarAt(mid);     //取出排序結果的中間節點    exm_set_copy.remove(mid);                               //將中間節點作為父（根）節點，所有將其從資料集中去除

    //確定左右節點    ExamplarSet exm_set_left(0, exm_set_copy.getDims());
    ExamplarSet exm_set_right(0, exm_set_copy.getDims());
    exm_set_right.remove(0);

    int size_new = exm_set_copy.getSize();         //獲得子資料空間大小    for(int i=0;i<size_new;i++)                    //生成左右子節點    {
        _Examplar temp = exm_set_copy[i];
        if( temp.dataAt(dim_max_var) < 
            exm_split.dataAt(dim_max_var) )
            exm_set_left.push_back(temp);
        else
            exm_set_right.push_back(temp);
    }

    KDTreeNode *pNewNode = new KDTreeNode(0, 0, 0, dim_max_var, exm_split, hr);
    pNewNode->_left_child = createKDTree(exm_set_left);        //遞迴呼叫生成左子樹    if(pNewNode->_left_child != NULL)                          //確認左子樹父節點        pNewNode->_left_child->_parent = pNewNode;            
    pNewNode->_right_child = createKDTree(exm_set_right);      //遞迴呼叫生成右子樹    if(pNewNode->_right_child != NULL)                         //確認右子樹父節點        pNewNode->_right_child->_parent = pNewNode;

    return pNewNode;    //最終返回k-d tree的根節點}