import java.util.*;
import java.io.*;

class HuffmanNode implements Comparable<HuffmanNode>{
HuffmanNode right;
HuffmanNode left;
HuffmanNode parent;
int count;          
String letter;

public HuffmanNode(){}

public HuffmanNode (String letter, int count){
this.letter = letter;
this.count = count;
}
public HuffmanNode (String letter, int count, HuffmanNode parent, HuffmanNode left, HuffmanNode right){
    this.letter = letter;
    this.count = count;
    this.left = left;
    this.right = right;
    this.parent = parent;
}

public void setCount(int count){
this.count = count;
}

public int getCount(){
return count;
}

public void setRight(HuffmanNode right){
this.right = right;
}

public HuffmanNode getRight(HuffmanNode right){
return right;
}

public void setLeft(HuffmanNode left){
this.left = left;
}

public HuffmanNode getLeft(HuffmanNode left){
return left;
}       
public void setParent(HuffmanNode right){
this.left = left;
}   
public HuffmanNode getParent(HuffmanNode parent){
return parent;
}

public void buildTree(HuffmanNode node){
    if (node.compareTo(this) <= 0 && left != null){
    System.out.println(node.getCount() + ":" + this.count);
    left.buildTree(node);
    }
    else if (node.compareTo(this) <= 0 && left == null){
    this.left = node;
    node.parent = this;
    }
    else if (node.compareTo(this) > 0 && right != null){
    System.out.println(node.getCount() + ":" +this.count);
    right.buildTree(node);
    }
    else if (node.compareTo(this) > 0 && right == null){
    this.right = node;
    node.parent = this;
    }
}


public int compareTo(HuffmanNode x){
return this.count - x.count;
}
public void genCode(String s){
    if(left != null){
    left.genCode(s + "0");
    }
    if(right != null){
    right.genCode(s + "1"); 
    }
    if (left == null && right == null){
    System.out.println(s);
    }
}
}

public class hw4{
public static void main (String []args)throws IOException{

//ask user to enter file name
System.out.printf("Enter a file location and name to encode [press Enter]: ");
Scanner input = new Scanner(System.in);
String filename = input.next();

//Gets file name from Scanner and checks to see if valid
File file = new File(filename);
//if (!file.isFile()){
//System.out.printf("Enter a file location and name to encode [press Enter]: ");
//}
Scanner text = new Scanner(file);

String[] letters = {"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
int[] freq = {0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0};

String letter;
String tempStr;
int tempInt;

    while(text.hasNext()){
        letter = text.next();
        //System.out.printf("%s\n", letter);

                    char c = letter.charAt(0);
        int index = c - 97;
        freq[index]++;     
    }

    for(int i=0; i <25; i++){
    System.out.printf("%s:%d\n", letters[i], freq[i]);
    }
    System.out.printf("\n");

    for (int n=0; n <25; n++) {
        for (int i=0; i <25; i++) {
            if (freq[i] > freq[i+1]) {
                // exchange elements
                tempInt = freq[i];  
                tempStr = letters[i]; 
                freq[i] = freq[i+1];
                letters[i] = letters[i+1];  
                freq[i+1] = tempInt;
                letters[i+1] = tempStr;
            }
        }   
       } 

    PriorityQueue<HuffmanNode> huffmanList = new PriorityQueue<HuffmanNode>();

    for(int i=0; i <26; i++){
    System.out.printf("%s:%d\n", letters[i], freq[i]);
        if(freq[i] > 0){
        huffmanList.add(new HuffmanNode(letters[i],freq[i]));
        }
    }

    HuffmanNode root = new HuffmanNode();

    while(huffmanList.size() > 1){
    HuffmanNode x = huffmanList.poll();
    HuffmanNode y = huffmanList.poll();
    HuffmanNode result = new HuffmanNode("-", x.getCount() + y.getCount(), null, x, y);
        if(root == null){
        root = result;
        }
        else{
        root.buildTree(result); 
        }
    huffmanList.add(result);                    
    }
     root.genCode(" ");
}
}

input file data:
a
a
a
a
d
d
d
d
d
d
d
d
k
k
k
k
k
k
f
f
f
f
f
f
h
h
h
h
h
h
b
b
b
b
b
b
b
b
n
n
n
n
n
n
n
e
e
e
e
e
i
i
i
i
i
i
i
i
l
k
j
a
n
s
g
l
k
j
a
s
v
o
i
j
a
s
d
l
k
g
k
n
m
a
s
d
k
l
o
v
h
a
s
d
g
z