总是报错，请大家帮下

老是报错，请大家帮下!
package   datastructure;
import   java.awt.*;
import   java.awt.geom.*;
import   java.awt.image.*;
import   java.io.*;
import   javax.imageio.*;
import   javax.servlet.*;
import   javax.servlet.http.*;
class   HTNode
{
int   weight;
int   parent;
int   lchild;
int   rchild;
}
public   class   HuffmanTree
{
private   int   pictureWidth=500;
        private   int   pictureHeight=500;
        public   int   s1,s2;
        int   n=(int)(2+9*Math.random());//随机生成结点的个数
int   m=2*n-1;  
public   void   draw(HttpServletResponse   response)  
{
response.reset();
response.setContentType( "image/png ");
BufferedImage   image=new   BufferedImage(pictureWidth,pictureHeight,BufferedImage.TYPE_INT_RGB);
Graphics2D   g2d=image.createGraphics();
g2d.setPaint(Color.WHITE);
g2d.fillRect(0,0,pictureWidth,   pictureHeight);
g2d.setPaint(Color.RED);
g2d.setFont(new   Font( "TimesRoman ",Font.BOLD,24));
g2d.drawString( "赫夫曼树和赫夫曼编码演示 ",100,20);
HTNode   HT[]=new   HTNode[m+1];
        HTNode   HC[]=new   HTNode[n];
HuffmanCoding(HT,HC,g2d);
g2d.dispose();
                ServletOutputStream   sos=null;
                try  
                {
                sos=response.getOutputStream();
                        ImageIO.write(image, "PNG ",sos);
                        sos.close();
                }  
                catch   (IOException   ex){
                }    
}
public   void   HuffmanCoding(HTNode   HT[],HTNode   HC[],Graphics2D   g2d)
{
int   i;
int   w[]=new   int[n];//存取每个结点的权值
for(i=0;i <n;i++)
w[i]=(int)(1+20*Math.random());
for(i=1;i <=n;i++)  
{    
HT[i].weight=w[i-1];  
        HT[i].parent=0;  
        HT[i].lchild=0;  
        HT[i].rchild=0;  
}  
for(i=n+1;i <=m;i++)  
{    
HT[i].weight=0;  
        HT[i].parent=0;  
        HT[i].lchild=0;  
        HT[i].rchild=0;  
}  
for(i=n+1;i <=m;i++)  
{  
Select(HT,i-1);  
        HT[s1].parent=i;  
        HT[s2].parent=i;  
        HT[i].lchild=s1;  
        HT[i].rchild=s2;  
        HT[i].weight=HT[s1].weight+   HT[s2].weight;  
}
for(i=1;i <=n;i++)
g2d.drawString(Integer.toString(HT[i].weight),20*i,100);
}
public   void   Select(HTNode   HT[],int   n)

{
int   i,j;  
for(i=1;i <=   n;i++)   if(HT[i].parent==0){s1=i;break;}  
for(j=i+1;j <=n;j++)  
if(HT[j].parent==0){s2=j;break;}  
for(i=1;i <=n;i++)  
if((HT[s1].weight> HT[i].weight)&&(HT[i].parent==0)&&(s2!=i))   s1=i;  
for(j=1;j <=   n;j++)  
if((HT[s2].weight> HT[j].weight)&&(HT[j].parent==0)&&(s1!=j))   s2=j;  
}  
}

为什么上面的程序老是报下面的错:
java.lang.NullPointerException
datastructure.HuffmanTree.HuffmanCoding(HuffmanTree.java:56)
datastructure.HuffmanTree.draw(HuffmanTree.java:36)
org.apache.jsp.WebRoot.huffmanTree_jsp._jspService(huffmanTree_jsp.java:61)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:324)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:292)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:236)
javax.servlet.http.HttpServlet.service(HttpServlet.java:802)



[解决办法]
没行号呀，看着累
[解决办法]
是啊 先确定是在哪一行附近，我们数不过来的
可能是取的数据是空指针，造成此类错误