Thread: Huffman Algorithm Help

I have been staring at this previous years exam question for hours now and I just can't get it working correctly. If anyone can help it would be greatly appreciated as I have an exam on Friday!

import java.util.*;
 
public class Huffman{
 
    public static void main(String[] args)
    {
       Scanner in = new Scanner(System.in);
 
       System.out.print("Enter your sentence: ");
       String sentence = in.nextLine();
       String binaryString="";      //this stores the string of binary code
 
 
       for(int i=0; i < sentence.length(); i++)
       {        
           int decimalValue = (int)sentence.charAt(i);      //convert to decimal
           String binaryValue = Integer.toBinaryString(decimalValue);      //convert to binary
           for(int j=7;j>binaryValue.length();j--)
           {
               binaryString+="0";           //this loop adds in those pesky leading zeroes
           }
 
           binaryString += binaryValue+" "; //add to the string of binary
       }
 
       System.out.println(binaryString);    //print out the binary
 
 
       int[] array = new int[256];      //an array to store all the frequencies
 
       for(int i=0; i < sentence.length(); i++)
       {    //go through the sentence
           array[(int)sentence.charAt(i)]++;    //increment the appropriate frequencies
 
       }
 
 
       PriorityQueue < Tree >  PQ = new PriorityQueue < Tree >() ;  //make a priority queue to hold the forest of trees     
 
       for(int i=0; i<array.length; i++)
       { //go through frequency array
           if(array[i]>0)
           { //print out non-zero frequencies - cast to a char
                System.out.println("'"+(char)i+"' appeared "+array[i]+((array[i] == 1) ? " time" : " times"));
 
 
                //FILL THIS IN:
 
                //MAKE THE FOREST OF TREES AND ADD THEM TO THE PQ
 
                //create a new Tree 
                //set the cumulative frequency of that Tree
                //insert the letter as the root node 
                //add the Tree into the PQ
 
 
           }
 
        }
 
 
 
        while(PQ.size()>1)
        {             
 
//FILL THIS IN:
 
            //IMPLEMENT THE HUFFMAN ALGORITHM
 
            //when you're making the new combined tree, don't forget to assign a default root node (or else you'll get a null pointer exception)
            //if you like, to check if everything is working so far, try printing out the letter of the roots of the two trees you're combining
 
        }
 
        Tree HuffmanTree = PQ.poll();   //now there's only one tree left - get its codes
 
 
        //FILL THIS IN:
 
        //get all the codes for the letters and print them out
        //call the getCode() method on the HuffmanTree Tree object for each letter in the sentence
 
        //print out all the info
 
    }      
 
 
public class Tree implements Comparable<Tree>
{
           public Node root;             // first node of tree
             public int frequency=0;
 
             public Tree()                  // constructor
             {  
            	 root = null; 
              }
 
             public int compareTo(Tree object)
             { //
                 if(frequency-object.frequency>0)
                 { //compare the cumulative frequencies of the tree
                     return 1;
                  }
                 else if(frequency-object.frequency<0)
                 {
                     return -1;   //return 1 or -1 depending on whether these frequencies are bigger or smaller
                  }
 
                 else
                 {
                      return 0;   //return 0 if they're the same
                  }
             }
 
 
             String path="error";
 
             public String getCode(char letter)
             {
            	//FILL THIS IN:
 
                 //How do you get the code for the letter? Maybe try a traversal of the tree
                 //Track the path along the way and store the current path when you arrive at the right letter
 
                 return path;     //return the path that results
 
             }
 
}
 
 
 
 
class Node
{
 
public char letter;            //stores letter
 
public Node leftChild;         // this node's left child
public Node rightChild;        // this node's right child
 
}  // end class Node
 
 
}

I have included the entire problem! Help with any part would be greatly appreciated!