Added java implementation of Huffman compression (#181)

* Implementation of Huffman compression in Java

* Update CONTRIBUTORS.md

* Update huffman.java

* Some of the requested changes.

I've made some of the requested changes.

* Update huffman.java

* Added java in huffman.md

Author: sklan

CONTRIBUTORS.md

@@ -5,10 +5,11 @@ Gathros
 Jeremie Gillet (- Jie -)
 Salim Khatib
 Hitesh C
+Shaurya
 Maxime Dherbécourt
 Jess 3Jane
 Pen Pal
 Chinmaya Mahesh
 Unlambder
 Kjetil Johannessen
-CDsigma
+CDsigma

chapters/data_compression/huffman/code/java/huffman.java

@@ -0,0 +1,113 @@
+import java.util.*;
+
+class TreeNode {
+    String letter = "";
+    int frequency = 0;
+    TreeNode left = null, right = null;
+
+    public TreeNode(String letter, int frequency) {
+        this.letter = letter;
+        this.frequency = frequency;
+    }
+
+    public TreeNode(int frequency, TreeNode left, TreeNode right) {
+        this.frequency = frequency;
+        this.left = left;
+        this.right = right;
+    }
+}
+
+class HuffmanTree {
+    private Map<String, Integer> frequencyMap = new HashMap<>();
+    private Map<String, String> codeBook = new HashMap<>(), reverseCodeBook = new HashMap<>();
+    private TreeNode root;
+    private String stringToEncode;
+
+    public HuffmanTree(String stringToEncode) {
+        this.stringToEncode = stringToEncode;
+    }
+
+    public void createTree() {
+        for (int i = 0; i < stringToEncode.length(); i++) {
+            String key = Character.toString(stringToEncode.charAt(i));
+            if (!frequencyMap.containsKey(key)) {
+                frequencyMap.put(key, 1);
+            } else {
+                int frequency = frequencyMap.get(key) + 1;
+                frequencyMap.replace(key, frequency);
+            }
+        }
+        Queue<TreeNode> priorityQueue = new PriorityQueue<>(Comparator.comparingInt(o -> o.frequency));
+        for (Map.Entry<String, Integer> m : frequencyMap.entrySet()) {
+            priorityQueue.add(new TreeNode(m.getKey(), m.getValue()));
+        }
+        while (priorityQueue.size() > 1) {
+            TreeNode left = priorityQueue.remove();
+            TreeNode right = priorityQueue.remove();
+            priorityQueue.add(new TreeNode(left.frequency + right.frequency, left, right));
+        }
+        root = priorityQueue.remove();
+    }
+
+    private void traverse(TreeNode node, StringBuilder code) {
+        if (node.left == null && node.right == null) {
+            codeBook.put(node.letter, code.toString());
+        }
+        if (node.left != null) {
+            traverse(node.left, code.append(0));
+            code.deleteCharAt(code.length() - 1);
+        }
+        if (node.right != null) {
+            traverse(node.right, code.append(1));
+            code.deleteCharAt(code.length() - 1);
+        }
+    }
+
+    public void printCodeBook() {
+        System.out.println("Code Book");
+        for (Map.Entry<String, String> m : codeBook.entrySet()) {
+            System.out.println(m.getKey() + "\t" + m.getValue());
+        }
+        System.out.println();
+    }
+
+    private void CodeBookReverse() {
+        for (Map.Entry<String, String> m : codeBook.entrySet()) {
+            reverseCodeBook.put(m.getValue(), m.getKey());
+        }
+    }
+
+    public String encode() {
+        traverse(root, new StringBuilder());
+        StringBuilder encode = new StringBuilder();
+        for (int i = 0; i < stringToEncode.length(); i++) {
+            String k = Character.toString(stringToEncode.charAt(i));
+            encode.append(codeBook.get(k));
+        }
+        printCodeBook();
+        return encode.toString();
+    }
+
+    public String decode(String encoded) {
+        StringBuilder decoded = new StringBuilder(), key = new StringBuilder();
+        CodeBookReverse();
+        for (int i = 0; i < encoded.length(); i++) {
+            key = key.append(encoded.charAt(i));
+            if (reverseCodeBook.containsKey(key.toString())) {
+                decoded.append(reverseCodeBook.get(key.toString()));
+                key = new StringBuilder();
+            }
+        }
+        return decoded.toString();
+    }
+}
+
+class Huffman {
+    public static void main(String[] args) {
+        HuffmanTree huffmanTree = new HuffmanTree("bibbity_bobbity");
+        huffmanTree.createTree();
+        String encoded = huffmanTree.encode();
+        System.out.println("Encoded String: " + encoded);
+        System.out.println("Decoded String: " + huffmanTree.decode(encoded));
+    }
+}

chapters/data_compression/huffman/huffman.md

@@ -84,9 +84,11 @@ Program.cs
 {% sample lang="js" %}
 ### JavaScript
 [import, lang:"javascript"](code/javascript/huffman.js)
+{% sample lang="java" %}
+### Java
+[import, lang:"java"](code/java/huffman.java)
 {% endmethod %}
 
-
 <script>
 MathJax.Hub.Queue(["Typeset",MathJax.Hub]);
 </script>