Add Huffman algorithm

Author: jcazevedo

src/main/java/com/packt/datastructuresandalg/lesson4/huffman/Huffman.java

@@ -0,0 +1,70 @@
+package com.packt.datastructuresandalg.lesson4.huffman;
+
+import java.util.Comparator;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.PriorityQueue;
+
+public class Huffman {
+    static class Node {
+        public Node left;
+        public Node right;
+        public Integer frequency;
+        public Character character;
+
+        public Node() {
+            left = right = null;
+            frequency = 0;
+            character = null;
+        }
+    }
+
+    Node build(Map<Character, Integer> frequencies) {
+        int N = frequencies.size();
+        PriorityQueue<Node> pq = new PriorityQueue<>(N,
+                (o1, o2) -> Integer.signum(o1.frequency - o2.frequency));
+        for (Map.Entry<Character, Integer> entry : frequencies.entrySet()) {
+            Node n = new Node();
+            n.character = entry.getKey();
+            n.frequency = entry.getValue();
+            pq.add(n);
+        }
+        for (int i = 0; i < N - 1; i++) {
+            Node n = new Node();
+            n.left = pq.remove();
+            n.right = pq.remove();
+            n.frequency = n.left.frequency + n.right.frequency;
+            pq.add(n);
+        }
+        return pq.remove();
+    }
+
+    public static String decode(String encoding, Node current, Node code, String s) {
+        if (current.character != null)
+            return decode(encoding, code, code, s + current.character);
+        if (encoding.isEmpty())
+            return s;
+        if (encoding.charAt(0) == '0')
+            return decode(encoding.substring(1), current.left, code, s);
+        return decode(encoding.substring(1), current.right, code, s);
+    }
+
+    public static String decode(String encoding, Node code) {
+        return decode(encoding, code, code, "");
+    }
+
+    public static void main(String[] args) {
+        Map<Character, Integer> frequencies = new HashMap<>();
+        frequencies.put('a', 45000);
+        frequencies.put('b', 13000);
+        frequencies.put('c', 12000);
+        frequencies.put('d', 16000);
+        frequencies.put('e', 9000);
+        frequencies.put('f', 5000);
+
+        Huffman huffman = new Huffman();
+        Node code = huffman.build(frequencies);
+
+        System.out.println(Huffman.decode("001011101", code));
+    }
+}