前缀树

static class Trie {
    private Trie[] subNodes;//子结点
    private boolean isEnd;//是否为字符串最后一个字符

    // 初始化前缀树
    public Trie() {
        this.subNodes = new Trie[26];//26个字符
        this.isEnd = false;
    }

    // 插入字符到树中。
    public void insert(String word) {
        Trie node = this;
        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            int index = c - 'a';
            if (node.subNodes[index] == null) {
                node.subNodes[index] = new Trie();
            }
            node = node.subNodes[index];
        }
        node.isEnd = true;
    }

    // 返回单词是否在trie中。
    public boolean search(String word) {
        Trie node = searchPrefix(word);
        return node != null && node.isEnd;
    }

    // 返回在trie中是否有以给定前缀开头的单词。 
    public boolean startsWith(String prefix) {
        return searchPrefix(prefix) != null;
    }

    public Trie searchPrefix(String prefix) {
        Trie node = this;
        for (int i = 0; i < prefix.length(); i++) {
            int index = prefix.charAt(i) - 'a';
            if (node.subNodes[index] == null) {
                return null;
            }
            node = node.subNodes[index];
        }
        return node;
    }


}