大家帮看下!

1. package datastructure;
2. import java.awt.*;
3. import java.awt.geom.*;
4. import java.awt.image.*;
5. import java.io.*;
6. import javax.imageio.*;
7. import javax.servlet.*;
8. import javax.servlet.http.*;
9. class HTNode
10.{
11. int weight;
12. int parent;
13. int lchild;
14. int rchild;
15.}
16. public class HuffmanTree
17.{
18. private int pictureWidth=500;
19.    private int pictureHeight=500;
20.    public int s1,s2;
21.    int n=(int)(2+9*Math.random());//随机生成结点的个数
22. int m=2*n-1; 
23. public void draw(HttpServletResponse response) 
24. {
25. response.reset();
26. response.setContentType("image/png");
27. BufferedImage image=new BufferedImage(pictureWidth,pictureHeight,BufferedImage.TYPE_INT_RGB);
28. Graphics2D g2d=image.createGraphics();
29. g2d.setPaint(Color.WHITE);
30. g2d.fillRect(0,0,pictureWidth, pictureHeight);
31. g2d.setPaint(Color.RED);
32. g2d.setFont(new Font("TimesRoman",Font.BOLD,24));
33. g2d.drawString("赫夫曼树和赫夫曼编码演示",100,20);
34. HTNode HT[]=new HTNode[m+1];
35.     HTNode HC[]=new HTNode[n];
36. HuffmanCoding(HT,HC,g2d);
37. g2d.dispose();
38.        ServletOutputStream sos=null;
39.        try 
40.        {
41.         sos=response.getOutputStream();
42.            ImageIO.write(image,"PNG",sos);
43.            sos.close();
44.        } 
45.        catch (IOException ex){
46.        }  
47. }
48. public void HuffmanCoding(HTNode HT[],HTNode HC[],Graphics2D g2d)
49. {
50. int i;
51. int w[]=new int[n];//存取每个结点的权值
52. for(i=0;i<n;i++)
53. w[i]=(int)(1+20*Math.random());
54. for(i=1;i<=n;i++) 
55. {  
56. 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)