|NO.Z.00042|——————————|BigDataEnd|——|Hadoop&ElasticSearch.V42|——|ELK.v42|原理剖析|数据结构.V2|

一、Trie(扩展)

### --- Trie 被称作做字典树、前缀树（Prefix Tree）、单词查找树

~~~     Trie 搜索字符串的效率主要跟字符串的长度有关(O(len(单词)))
~~~     使用 Trie 存储 cat->1、dog->2、doggy->3、does->4、cast->5、add->6 六个单词映射

### --- Trie时间复杂度：O(len(key))

~~~     FST，不但能共享前缀还能共享后缀。不但能判断查找的key是否存在，还能给出响应的输入output。
~~~     它在时间复杂度和空间复杂度上都做了最大程度的优化，
~~~     使得Lucene能够将Term Index完全加载到内存，
~~~     快速的定位Term找到响应的output（posting倒排列表）。

二、编程代码实现

### --- 相关方法

~~~     # 相关方法
    int size();
    boolean isEmpty();
    void clear();
    boolean contains(String str);
    V add(String str, V value);
    V remove(String str);
    boolean starsWith(String prefix);

### --- 编程实现

package com.yanqi.es;

import java.util.HashMap;

//指明trie存储value的泛型，具体类型是创建时指定
public class Trie<E> {

    //size属性
    private int size;
    //根节点
    private Node<E> root;
    //当前存储的单词容量
    int size() {
        return size;
    };

    //判断当前是否有数据存储
    boolean isEmpty() {
        return size == 0;
    };

    //清空数据
    void clear() {
        size = 0;
        root = null;
    };

    //是否包含某个单词
    boolean contains(String str) { final Node<E> node = search(str);
    return node != null && node.word;
    }

    //检查key是否合法
    public void checkKeyIsNull(String key) {
        //判断单词是否合法，null ,空字符串是不合适的
        if (null == key || key.length() == 0) {
            //抛出异常
            throw new IllegalArgumentException("key should not bu null..");
        }
    }

    //添加一个单词--》value,返回原理的old值
    // add("a",1)
    // add("a",2)
    E add(String str, E value) { checkKeyIsNull(str); //key不合法抛出异常，方法停止运行

        // 获取到根节点
        if (root == null) {
            root = new Node<>(null);
        }
        Node<E> node = root;
        //添加cat -->1 遍历key
        int len = str.length();
        for (int i = 0; i < len; i++) {
            //获取字母 c a t --->value
            char c = str.charAt(i);
            //判断childs是否为空
            HashMap<Character, Node<E>> childsMap = node.childs == null ? new HashMap<>() : node.childs;
            //把childsmap赋值给node.childs
            node.childs = childsMap;
            //根据c去当前节点的childs属性中判断是否有向下的分支
            Node<E> childNode = childsMap.get(c);
            if (childNode == null) {
                //创建一个新的节点
                childNode = new Node<>(node);
                //设置character属性
                childNode.character = c;
                //回填到父节点的childs中
                childsMap.put(c, childNode);
            }
            //节点已经存在
            node = childNode;
        }
        //原来是否有这个单词，覆盖value
        if (node.word) {
            //说明之前存在该单词
            E oldValue = node.value;
            node.value = value;
            return oldValue;
        }

        //新增一个单词
        node.word = true;
        node.value = value;
        size++;
        return null;
    }

    //删除一个单词 返回原来的value
    E remove(String str) {
        //首先找到最后一个节点
        Node<E> node = search(str);
        if (node == null || !node.word) {
            return null;
        }
        //开始删除动作
        size--;
        final E oldValue = node.value;
        //如果还有另外的子节点
        if (node.childs != null && !node.childs.isEmpty()) {
            node.word = false;
            node.value = null;
            // node.childs=null; 不能设置null
            return oldValue;
        }
        //没有子节点
        Node<E> parent = null;
        while ((parent = node.parent) != null) {
            parent.childs.remove(node.character);
            if (!parent.childs.isEmpty() || parent.word) {
                break;
            }
            node = parent;
        }
        return oldValue;
    }
    //在trie中搜索某个字符串
    private Node<E> search(String key) {
        //校验key
        checkKeyIsNull(key);
        //遍历输入的key，查找最后一个字母的节点
        final int len = key.length();
        Node<E> node = root;
        for (int i = 0; i < len; i++) {
            if (node == null || node.childs == null || node.childs.isEmpty()) {
                return null;
            }
            node = node.childs.get(key.charAt(i));
        }
        //node有可能为null
        return node;
    }
    //判断是否有某个前缀开头的单词
    boolean starsWith(String prefix) {
        final Node<E> node = search(prefix);
        return node != null;
    };

    //节点类
    private static class Node<E> {
        //value属性
        E value;
        //是否存储了某个单词
        boolean word;
        //分支信息的存储:character可以存储字母也可以是中文一个字
        HashMap<Character, Node<E>> childs;
        //父节点
        Node<E> parent;
        //当前节点character属性
        Character character;
        //构造函数
        public Node(Node<E> parent) {
            this.parent = parent;
        }
    }
}

### --- 编程实现测试类

package com.yanqi.es;

public class TrieTest {
    public static void main(String[] args) {
        final Trie<Integer> trie = new Trie<>();
        //返回原来的value值
        final Integer cat = trie.add("cat", 10);
        // System.out.println(cat);
        final Integer dog = trie.add("dog", 20);
        // System.out.println(dog);
        final Integer newCat = trie.add("cat", 100);
        // System.out.println(newCat);
        //
        // System.out.println(trie.contains("cat"));
        // System.out.println(trie.starsWith("do"));
        // System.out.println(trie.starsWith("a"));
        final Integer res = trie.remove("cat");
        System.out.println(res);
        System.out.println(trie.contains("cat"));
        System.out.println(trie.remove("doggy"));
    }
}

### --- 编译打印

D:\JAVA\jdk1.8.0_231\bin\java.exe "-javaagent:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\lib\idea_rt.jar=58442:D:\IntelliJIDEA\IntelliJ IDEA 2019.3.3\bin" -Dfile.encoding=UTF-8 -classpath 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com.yanqi.es.TrieTest
100
false
null

Process finished with exit code 0

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Walter Savage Landor:strove with none,for none was worth my strife.Nature I loved and, next to Nature, Art:I warm'd both hands before the fire of life.It sinks, and I am ready to depart

                                                                                                                                                   ——W.S.Landor

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