Trie

public class Trie {
    private class Node {
        // 判断是否是一个单词，我们不能只依靠叶子节点来判断是否结束
        boolean isWord;
        // 边与节点的映射
        Map<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;
    }

    // 获取单词数量
    public int getSize() {
        return size;
    }

    // 向Trie中添加新的单词word
    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++;
        }
    }

    public boolean constains(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);
        }
        // 返回这个单词是否包含
        return cur.isWord;
    }
    // 查询是否存在Trie中有单词以prefix为前缀
    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;
    } 
}