Java Trie(词典树)实现

实现Trie tree，可用作实现词典。可用来存储，查找及删除string， 同时实现返回前缀为指定字符所有结果的功能。

每个node存所有child节点与及对应path上的字符所组成的map，利用count来记录每个节点的子树种存在多少word，便于删除的操作。

findAllWithPrefix 用到了DFS的思想，遍历所有带有前缀的结果并输出。常见应用于搜索引擎中。

class TrieNode {
    int count;
    Map<Character, TrieNode> children;
    boolean isWord;
    public TrieNode() {
        count=0;
        isWord=false;
        children=new HashMap<Character,TrieNode>();
    }
}

public class MyTrie {
    TrieNode root;

    public MyTrie() {
        root = new TrieNode();
    }

    public boolean search(String s) {
        if (this.root == null || s == null || s.length() == 0) {
            return false;
        }
        TrieNode cur=root;
        for (int i = 0; i < s.length(); i++) {
            TrieNode next=cur.children.get(s.charAt(i));
            if(next == null) {
                return false;
            }
            cur=next;
        }
        return cur.isWord;
    }
    public void insert(String s) {
        if(search(s) || s == null || s.length() == 0) {
            return;
        }
        TrieNode cur=root;
        for (int i = 0; i < s.length(); i++) {
            TrieNode next=cur.children.get(s.charAt(i));
            if(next == null) {
                  next=new TrieNode();
                  cur.children.put(s.charAt(i), next);
            }
            cur=next;
            cur.count++;
        }
        cur.isWord=true;
    }
    public boolean delete(String s) {
        if(!search(s)) {
            return false;
        }
        TrieNode cur=root;
        for(int i=0; i<s.length(); i++) {
            TrieNode next=cur.children.get(s.charAt(i));
            if(next.count == 1) {
                cur.children.remove(s.charAt(i));
                return true;
            }
            next.count--;
            cur=next;
        }
        cur.isWord=false;
        return true;
    }
    
    public List<String>findAllWithPrefix(String s){
        TrieNode matchNode=searchNode(s);
        if(matchNode == null) {
            return null;
        }
        List<String>result=new ArrayList<String>();
        DFS(result, matchNode, new StringBuilder(s));
        return result;
    }
    private void DFS(List<String>result, TrieNode matchNode, StringBuilder sb) {
        if(matchNode.isWord == true) {
            result.add(sb.toString());
        }
        for(Map.Entry<Character, TrieNode>child : matchNode.children.entrySet()) {
            sb.append(child.getKey());
            DFS(result,child.getValue(),sb);
            sb.deleteCharAt(sb.length()-1);
        }
    }
    private TrieNode searchNode(String s) {
        if(s == null || s.length() == 0) {
            return null;
        }
        TrieNode cur=root;
        for(int i=0; i<s.length(); i++) {
            TrieNode next=cur.children.get(s.charAt(i));
            if(next == null) {
                return cur;
            }
            cur=next;
        }
        return cur;
    }
}