[LeetCode]Word Search II

public class Solution {
    
    private char[][] board;
    private List<String> result = new ArrayList<String>();
    
    public List<String> findWords(char[][] board, String[] words) {
        TrieNode root = new TrieNode();
        for (String str : words) {
            buildTrie(str, root);
        }
        int row = board.length;
        if (row == 0) {
            return result;
        }
        int col = board[0].length;
        this.board = board;
        for (int r = 0; r < row; r++) {
            for (int c = 0; c < col; c++) {
                if (root.child[(int)board[r][c] - (int)'a'] != null)
                    find(r, c, "", root.child[(int)board[r][c] - (int)'a']);
            }
        }
        return result;
    }
    
    private void find(int r, int c, String str, TrieNode root) {
        if (root.isWord) {
            if (!result.contains(str + board[r][c]))
            result.add(str + board[r][c]);
        }
        char tmp = board[r][c];
        board[r][c] = '*';
        str = str + tmp;
        if (r > 0 && board[r - 1][c] != '*' && root.child[board[r - 1][c] - 'a'] != null) {
            find(r - 1, c, str, root.child[(int)board[r - 1][c] - (int)'a']);
        }
        if (r + 1 < board.length && board[r + 1][c] != '*' && root.child[board[r + 1][c] - 'a'] != null) {
            find(r + 1, c, str, root.child[(int)board[r + 1][c] - (int)'a']);
        }
        if (c > 0 && board[r][c - 1] != '*' && root.child[board[r][c - 1] - 'a'] != null) {
            find(r, c - 1, str, root.child[(int)board[r][c - 1] - (int)'a']);
        }
        if (c + 1 < board[0].length && board[r][c + 1] != '*' && root.child[board[r][c + 1] - 'a'] != null) {
            find(r, c + 1, str, root.child[(int)board[r][c + 1] - (int)'a']);
        }
        board[r][c] = tmp;
    }
    private void buildTrie (String word, TrieNode root) {
        if (word.length() == 0) {
            root.isWord = true;
            return;
        }
        char tmp = word.charAt(0);
        if (root.child[(int)tmp - (int)'a'] == null) {
            root.child[(int)tmp - (int)'a'] = new TrieNode();
        }
        buildTrie(word.substring(1), root.child[(int)tmp - (int)'a']);
    }
    class TrieNode {
        TrieNode[] child = new TrieNode[26];
        boolean isWord;
        TrieNode() {
            isWord = false;
        }
    }
}