Word Search II Backtracking with Trie

public List<String> findWords(char[][] board, String[] words) {
    Trie trie = buildTrie(words);
    Set<String> res = new HashSet<>();
    for (int i = 0; i < board.length; i++) {
        for (int j = 0; j < board[0].length; j++) {
            dfs(board, trie, res, i, j);
        }
    }
    return new ArrayList<>(res);
}

public void dfs(char[][] board, Trie node, Set<String> res, int i, int j) {
    if (i < 0 || i >= board.length || j < 0 || j >= board[0].length || 
        board[i][j] == '#' || node.next[board[i][j] - 'a'] == null) {
            return;
    }
    if (node.next[board[i][j] - 'a'].word != null) {
        res.add(node.next[board[i][j] - 'a'].word);
    }

    // Go to next char
    node = node.next[board[i][j] - 'a']; 
    char c = board[i][j];
    board[i][j] = '#';
    dfs(board, node, res, i - 1, j);
    dfs(board, node, res, i + 1, j);
    dfs(board, node, res, i, j - 1);
    dfs(board, node, res, i, j + 1);
    board[i][j] = c;
}   

public Trie buildTrie(String[] words) {
    Trie root = new Trie();
    for (String w : words) {
        Trie p = root;
        for (char c : w.toCharArray()) {
            if (p.next[c - 'a'] == null) {
                p.next[c - 'a'] = new Trie();
            }
            p = p.next[c - 'a'];  // will point to curr char
        }
        p.word = w;
    }
    return root;
}

private class Trie {
    Trie[] next = new Trie[26];
    String word = null;
}