区间第K大

    protected static int partitions(List<KDNode> data,int left,int right,int k,int pos){
        int l = left,r = right;
        if(l==r) return l;
        KDNode key = data.get(l);
        while (l < r){
            while(l < r && data.get(r).getVal(pos) > key.getVal(pos) ) r --;
            data.set(l,data.get(r));
            while(l < r && data.get(l).getVal(pos) < key.getVal(pos) ) l ++;
            data.set(r,data.get(l));
        }
        data.set(l,key);
        if ( l == k ) return l;
        if ( l < k )
            return partitions(data,l+1,right,k,pos);
        if ( l > k )
            return partitions(data,left,l-1,k,pos);
        return -1;
    }