Implement Trie (Prefix Tree)

Implement a trie with insert, search, and startsWith methods.

Note:
You may assume that all inputs are consist of lowercase letters a-z.

思路：
之前也没有接触过Trie，百科上查了一下，大概就是词源的问题，N个word有公共前缀，只是后缀不同，可以用树表示。
可以通过递归解决。

代码如下：

class TrieNode {    // 应该都是private的;只是为了减少代码量    // Initialize your data structure here.    public Map<Character,TrieNode> map;//存放后缀    public char val;//当前节点的字符值    public boolean tail;//一个字符串以该节点结尾    public TrieNode() {        map = new HashMap<>();        tail=false;    }    public TrieNode(char c) {        map = new HashMap<>();        this.val=c;        tail=false;    }}public class Trie {    private TrieNode root;    public Trie() {        root = new TrieNode();    }    // Inserts a word into the trie.    public void insert(String word) {        insert(root,word);    }    private void insert(TrieNode root,String word){        if(word.length()==0){            root.tail=true;            return;        }        TrieNode cur=root;        char c=word.charAt(0);        if(cur.map.containsKey(c)){            TrieNode child = cur.map.get(c);            insert(child,word.substring(1));        }else{            TrieNode child = new TrieNode(c);            cur.map.put(c,child);            insert(child,word.substring(1));        }    }    // Returns if the word is in the trie.    public boolean search(String word) {        return search(root,word);    }    private boolean search(TrieNode root,String word){        if(word.length()==0)            return root.tail==true;        char c = word.charAt(0);        if(!root.map.containsKey(c))            return false;        TrieNode child = root.map.get(c);        return search(child,word.substring(1));    }    // Returns if there is any word in the trie    // that starts with the given prefix.    public boolean startsWith(String prefix) {         return startsWith(root,prefix);    }    private boolean startsWith(TrieNode root,String prefix) {        if(prefix.length()==0)            return true;        char c = prefix.charAt(0);        if(!root.map.containsKey(c))            return false;        TrieNode child = root.map.get(c);        return startsWith(child,prefix.substring(1));    }}// Your Trie object will be instantiated and called as such:// Trie trie = new Trie();// trie.insert("somestring");// trie.search("key");