实现 Trie (前缀树)

class Trie {
    class TrieNode {
        private boolean isEnd;
        private TrieNode[] links;

        TrieNode() {
            links = new TrieNode[26];
        }

        public boolean containsKey(char ch) {
            return links[ch-'a'] != null;
        }

        public TrieNode get(char ch) {
            return links[ch-'a'];
        }

        public void put(char ch, TrieNode node) {
            links[ch-'a'] = node;
        }

        public void setEnd() {
            isEnd = true;
        }

        public boolean isEnd() {
            return isEnd;
        }
    }

    private TrieNode root;

    /** Initialize your data structure here. */
    public Trie() {
        root = new TrieNode();
    }
    
    /** Inserts a word into the trie. */
    public void insert(String word) {
        TrieNode node = root;
        for(char ch : word.toCharArray()) {
            if(!node.containsKey(ch)) {
                node.put(ch, new TrieNode());
            }
            node = node.get(ch);
        }
        node.setEnd();
    }
    
    /** Returns if the word is in the trie. */
    public boolean search(String word) {
        TrieNode node = root;
        for(char ch : word.toCharArray()) {
            if(!node.containsKey(ch)) {
                return false;
            }
            node = node.get(ch);
        }
        return node.isEnd();
    }
    
    /** Returns if there is any word in the trie that starts with the given prefix. */
    public boolean startsWith(String prefix) {
        TrieNode node = root;
        for(char ch : prefix.toCharArray()) {
            if(!node.containsKey(ch)) {
                return false;
            }
            node = node.get(ch);
        }
        return true;
    }
}

/**
 * Your Trie object will be instantiated and called as such:
 * Trie obj = new Trie();
 * obj.insert(word);
 * boolean param_2 = obj.search(word);
 * boolean param_3 = obj.startsWith(prefix);
 */