preOrder.java

    preOrder(Trie curr, List<String> sortedWords)
    {
        if (curr == null || sortedWords.size() == 3 ) {
            return;
        }

        for (int i = 0; i < 26; i++)
        {
            if (curr.character[i] != null)
            {
                if (curr.character[i].key != null) {
                    sortedWords.add(curr.character[i].key);
                }
                preOrder(curr.character[i], sortedWords);
            }
        }
    }

suggestedWords.java

    List<List<String>> suggestedSearchWord(String[] products, String searchWord) {
        List<List<String>> suggests = new ArrayList<>();
        Trie root = new Trie();
        for (String product : products) {
            root.insert(root, product);
        }
        Trie curr = root;
        for (int i = 0; i < searchWord.length(); i++) {
            if (curr != null) {
                curr = curr.search(String.valueOf(searchWord.charAt(i)));
                List<String> sortedWords = new ArrayList<>();
                if (curr != null && curr.key != null)
                    sortedWords.add(curr.key);
                preOrder(curr, sortedWords);
                suggests.add(sortedWords);
            }
            else
                break;
        }
        return suggests;
    }

Trie.java

class Trie {
    String key;
    Trie[] character = null;

    // Constructor
    Trie()
    {
        // Trie supports lowercase English characters (a - z)
        // so character size is 26
        character = new Trie[26];
    }

    public  void insert(Trie head, String str)
    {
        // start from root node
        Trie curr = head;
        for (int i = 0; i < str.length(); i++)
        {
            // create a new node if path doesn't exists
            if (curr.character[str.charAt(i) - 'a'] == null) {
                curr.character[str.charAt(i) - 'a'] = new Trie();
            }
            // go to next node
            curr = curr.character[str.charAt(i) - 'a'];
        }

        // store key in leaf node
        curr.key = str;
    }


    public Trie search(String key) {
        return character[key.charAt(0) - 'a'];
    }
}