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!

Can you explain what the problem is?
Post any error messages
or the program's output and add some comments describing what's wrong.

Do you have the algorithm that the code is supposed to implement? You need that before you can write any code.

Do you have the algorithm that the code is supposed to implement? You need that before you can write any code.