212. Word Search II

class TrieNode{
    char val;
    TrieNode[] children;
    String word;
    
    public TrieNode(char x){
        children = new TrieNode[26];
        word = null;
    }
}
class Solution {
    public List<String> findWords(char[][] board, String[] words) {
        List<String> res = new ArrayList<>();
        if(board == null || board.length == 0) return res;
        TrieNode root = new TrieNode(' ');
        buildTrie(root, words);
        for(int i = 0; i < board.length; i++){
            for(int j = 0; j < board[0].length; j++){
                char c = board[i][j];
                if(root.children[c - 'a'] != null){
                    dfs(board, i, j, root, res);
                }
            }
        }
        return res;
    }
    
    private void buildTrie(TrieNode root, String[] words){
        for(String s : words){
            TrieNode cur = root;
            for(char c : s.toCharArray()){
                if(cur.children[c - 'a'] == null){
                    cur.children[c - 'a'] = new TrieNode(c);
                }
                cur = cur.children[c - 'a'];
            }
            cur.word = s;
        }
    }
    
    private void dfs(char[][] board, int i, int j, TrieNode cur, List<String> res){
        if(i < 0 || i >= board.length || j < 0 || j >= board[0].length) return;
        
        char c = board[i][j];
        if(c == '*') return;
        if(cur.children[c - 'a'] == null) return;
        
        cur = cur.children[c - 'a'];
        if(cur.word != null){
            res.add(cur.word);
            cur.word = null;
        }
        
        board[i][j] = '*';
        dfs(board, i + 1, j, cur, res);
        dfs(board, i - 1, j, cur, res);
        dfs(board, i, j + 1, cur, res);
        dfs(board, i, j - 1, cur, res);
        board[i][j] = c;
    }
}