Trie树

字典树，即Trie树，又称单词查找树或键树，是一种树形结构。典型应用是用于统计和排序大量的字符串（但不仅限于字符串），所以经常被搜索引擎系统用于文本词频统计。它的优点是：最大限度地减少无谓的字符串比较，查询效率比哈希表高。

//leetcode submit region begin(Prohibit modification and deletion)
class Trie {
    TrieNode root;
    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode cur =root;
        char c;
        for(int i = 0;i<word.length();i++){
            c =word.charAt(i);
            if(cur.get(c)!=null){
                cur= cur.get(c);
            }else {
                cur.put(c,new TrieNode());
                cur= cur.get(c);
            }
        }
        cur.setEnd();
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode cur = root;
        char c;
        for(int i=0;i<word.length();i++){
            c = word.charAt(i);
            if(cur.get(c)==null)
                return false;
            cur=cur.get(c);
        }
         return  cur.isEnd==true?true:false;
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode cur = root;
        char c;
        for(int i=0;i<prefix.length();i++){
            c = prefix.charAt(i);
            if(cur.get(c)==null)
                return false;
            cur=cur.get(c);
        }
        return true;
    }
}
class TrieNode{

    Map <Character, TrieNode> nextTreeNode;
    boolean isEnd;

    public TrieNode(){
        nextTreeNode = new HashMap<>();
        isEnd = false;
    }

    public boolean containsKey(char c){
        return  nextTreeNode.containsKey(c);
    }

    public TrieNode get(char c){
        return  nextTreeNode.get(c);
    }

    public void put(char c,TrieNode node){
        nextTreeNode.put(c,node);
    }

    public boolean isEnd(){
        return  isEnd;
    }

    public void setEnd(){
        isEnd = true;
    }
}