208. Implement Trie (Prefix Tree)

class Trie {

    private static class TrieNode {
        private final TrieNode[] children;
        private boolean end;

        public TrieNode() {
            this.children = new TrieNode[26];
        }
    }

    private final TrieNode root;

    /**
     * Initialize your data structure here.
     */
    public Trie() {
        this.root = new TrieNode();
    }

    /**
     * Inserts a word into the trie.
     */
    public void insert(String word) {
        TrieNode node = root;
        for (int i = 0; i < word.length(); i++) {
            int offset = word.charAt(i) - 'a';
            if (node.children[offset] == null) {
                node.children[offset] = new TrieNode();
            }
            node = node.children[offset];
        }
        node.end = true;
    }

    /**
     * Returns if the word is in the trie.
     */
    public boolean search(String word) {
        TrieNode node = getNodeByPrefix(word);
        return node != null && node.end;
    }

    private TrieNode getNodeByPrefix(String prefix) {
        TrieNode node = root;
        for (int i = 0; i < prefix.length(); i++) {
            int offset = prefix.charAt(i) - 'a';
            if (node.children[offset] == null) {
                return null;
            }
            node = node.children[offset];
        }
        return node;
    }

    /**
     * Returns if there is any word in the trie that starts with the given prefix.
     */
    public boolean startsWith(String prefix) {
        return getNodeByPrefix(prefix) != null;
    }

}

Reference

208. Implement Trie (Prefix Tree)
剑指 Offer II 062. 实现前缀树