Thinking process

class TrieNode {
    public TrieNode[] children;
    public boolean isWord;

    public TrieNode(){
        children = new TrieNode[26];
        for (int i = 0; i < 26; i++) {
            children[i] = null;
        }
        isWord = false;
    }
}

class Trie {
    public TrieNode root;

    public Trie(){
        root = new TrieNode();
    }

    public void insert(String word) {
        TrieNode node = root;
        for (char c : word.toCharArray()) {
            if (node.children[c - 'a'] == null) {
                TrieNode child = new TrieNode();
                node.children[c - 'a'] = child;
            }
            node = node.children[c - 'a'];
        }
        node.isWord = true;
    }

}

public class Solution {
    /**
     * @param board: A list of lists of character
     * @param words: A list of string
     * @return: A list of string
     */
    public List<String> findWords(char[][] board, String[] words) {
        //init
        List<String> res = new ArrayList<>();
        Trie trie = new Trie();
        //convert words to trie
        for (String word : words) {
            trie.insert(word);
        }

        StringBuilder sb = new StringBuilder();
        //scan board
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                boolean[][] visited = new boolean[board.length][board[0].length];
                search(board, trie.root, visited, res, sb, i, j);
            }
        }

        return res;
    }

    private void search(char[][] board, TrieNode node, boolean[][] visited, List<String> res, StringBuilder sb, int i, int j) {
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length || node == null || visited[i][j]) return;

        visited[i][j] = true;
        sb.append(board[i][j]);

        TrieNode next = node.children[board[i][j] - 'a'];
        if (next != null && next.isWord) {
            if (!res.contains(sb.toString())) {
                res.add(sb.toString());
            }
        }

        search(board, next, visited, res, sb, i - 1, j);//up
        search(board, next, visited, res, sb, i + 1, j);//down
        search(board, next, visited, res, sb, i, j - 1);//left
        search(board, next, visited, res, sb, i, j + 1);//right
        visited[i][j] = false;
        sb.deleteCharAt(sb.length() - 1);
    }
}