Huffman.java

package com.thealgorithms.others;

import java.util.Comparator;
import java.util.PriorityQueue;
import java.util.Scanner;

// node class is the basic structure
// of each node present in the Huffman - tree.
class HuffmanNode {

    int data;
    char c;

    HuffmanNode left;
    HuffmanNode right;
}

// comparator class helps to compare the node
// on the basis of one of its attribute.
// Here we will be compared
// on the basis of data values of the nodes.
class MyComparator implements Comparator<HuffmanNode> {

    public int compare(HuffmanNode x, HuffmanNode y) {
        return x.data - y.data;
    }
}

public final class Huffman {
    private Huffman() {
    }

    // recursive function to print the
    // huffman-code through the tree traversal.
    // Here s is the huffman - code generated.
    public static void printCode(HuffmanNode root, String s) {
        // base case; if the left and right are null
        // then its a leaf node and we print
        // the code s generated by traversing the tree.
        if (root.left == null && root.right == null && Character.isLetter(root.c)) {
            // c is the character in the node
            System.out.println(root.c + ":" + s);

            return;
        }

        // if we go to left then add "0" to the code.
        // if we go to the right add"1" to the code.
        // recursive calls for left and
        // right sub-tree of the generated tree.
        printCode(root.left, s + "0");
        printCode(root.right, s + "1");
    }

    // main function
    public static void main(String[] args) {
        Scanner s = new Scanner(System.in);

        // number of characters.
        int n = 6;
        char[] charArray = {'a', 'b', 'c', 'd', 'e', 'f'};
        int[] charfreq = {5, 9, 12, 13, 16, 45};

        // creating a priority queue q.
        // makes a min-priority queue(min-heap).
        PriorityQueue<HuffmanNode> q = new PriorityQueue<HuffmanNode>(n, new MyComparator());

        for (int i = 0; i < n; i++) {
            // creating a Huffman node object
            // and add it to the priority queue.
            HuffmanNode hn = new HuffmanNode();

            hn.c = charArray[i];
            hn.data = charfreq[i];

            hn.left = null;
            hn.right = null;

            // add functions adds
            // the huffman node to the queue.
            q.add(hn);
        }

        // create a root node
        HuffmanNode root = null;

        // Here we will extract the two minimum value
        // from the heap each time until
        // its size reduces to 1, extract until
        // all the nodes are extracted.
        while (q.size() > 1) {
            // first min extract.
            HuffmanNode x = q.peek();
            q.poll();

            // second min extarct.
            HuffmanNode y = q.peek();
            q.poll();

            // new node f which is equal
            HuffmanNode f = new HuffmanNode();

            // to the sum of the frequency of the two nodes
            // assigning values to the f node.
            f.data = x.data + y.data;
            f.c = '-';

            // first extracted node as left child.
            f.left = x;

            // second extracted node as the right child.
            f.right = y;

            // marking the f node as the root node.
            root = f;

            // add this node to the priority-queue.
            q.add(f);
        }

        // print the codes by traversing the tree
        printCode(root, "");
        s.close();
    }
}