数据结构与算法-树-赫夫曼树（赫夫曼编码）

package tree;

import java.util.*;


public class HuffmanTreeCode {

    public static void main(String[] args) {
        String s = "i like like like java do you like a java";
        char[] chars = s.toCharArray();

        //统计每个字符出现的次数
        Map count = new HashMap<>();
        for (char a : chars) {
            count.merge((byte) a, 1, Integer::sum);
        }

        List list = new ArrayList<>();
        for (Map.Entry map : count.entrySet()) {
            list.add(new Node(map.getKey(), map.getValue()));
        }

        Node node = createHuffmanTree(list);
        node.preOrder();
    }


    
    public static class Node implements Comparable {
        public Node left;
        public Node right;
        public Byte data;
        public int val;

        public Node(Byte data, int val) {
            this.data = data;
            this.val = val;
        }


        @Override
        public String toString() {
            return "Node{" +
                    "data=" + data +
                    ", val=" + val +
                    '}';
        }

        
        public void preOrder() {
            System.out.println(this);
            if (this.left != null) {
                this.left.preOrder();
            }
            if (this.right != null) {
                this.right.preOrder();
            }
        }

        @Override
        public int compareTo(Node o) {
            return this.val - o.val;
        }
    }

    
    public static Node createHuffmanTree(List list) {
        //当list只有一个元素代表赫夫曼树已经创建好了
        //每次将节点排好序，取前两个构成新的节点，移除原来的前两个节点，
        //将新的节点添加进list。最终只剩下一颗赫夫曼树
        while (list.size() > 1) {
            Collections.sort(list);
            Node leftNode = list.get(0);
            Node rightNode = list.get(1);
            Node parent = new Node(null, leftNode.val + rightNode.val);
            parent.left = leftNode;
            parent.right = rightNode;
            list.remove(leftNode);
            list.remove(rightNode);
            list.add(parent);
        }
        return list.get(0);
    }
}