java——字典树 Trie

字典树是一种前缀树

package Trie;
import java.util.TreeMap;

public class Trie {

    private class Node{
        public boolean isWord;
        public TreeMap<Character, Node> next;
        
        public Node(boolean isWord) {
            this.isWord = isWord;
            next = new TreeMap<>();
        }
        
        public Node() {
            this(false);
        }
    }
    
    private Node root;
    private int size;
    
    public Trie() {
        root = new Node();
        size = 0;
    }
    
    //获取Trie中存储的单词数量
    public int getSize() {
        return size;
    }
    
    public void add(String word) {
        Node cur = root;
        for(int i = 0 ; i < word.length() ; i ++) {
            char c = word.charAt(i);
            if(cur.next.get(c) == null) {
                cur.next.put(c, new Node());    
            }
            cur = cur.next.get(c);
        }
        if(!cur.isWord) {
            cur.isWord = true;
            size ++;
        }
    }
    
    //查询单词word四会否在Trie中
    public boolean contains(String word) {
        
        Node cur = root;
        for(int i = 0 ; i < word.length() ; i ++) {
            char c = word.charAt(i);
            if(cur.next.get(c) == null)
                return false;
            cur = cur.next.get(c);
        }
        return cur.isWord;
    }
    
    //前缀搜索
    public boolean isPrefix(String prefix) {
        
        Node cur = root;
        for(int i = 0 ; i < prefix.length() ; i ++) {
            char c = prefix.charAt(i);
            if(cur.next.get(c) == null) {
                return false;
            }
            cur = cur.next.get(c);
        }
        return true;
    }
}

节省空间：

　　压缩字典树，但是维护成本更高

　　三分搜索树