212. Word Search II

class Solution {
    class Trie{
        public Trie[] children=new Trie[26];
        public boolean isWord=false;
        public boolean added=false;
        public void insert(String word) {
            if(word.length()==0)
            {
                isWord=true;
                return;
            }
            int idx=word.charAt(0)-'a';
            if(children[idx]==null)
                children[idx]=new Trie();
            children[idx].insert(word.substring(1));
        }
    }
    public List<String> findWords(char[][] board, String[] words) {
        List<String> res=new ArrayList<String>();
        Trie root=new Trie();
        for(String word:words)
            root.insert(word);
        boolean[][] visited=new boolean[board.length][board[0].length];
        for(int i=0;i<board.length;i++)
            for(int j=0;j<board[0].length;j++)
                search(i ,j ,"" , root, visited, board, res);
        return res;
    }
    private void search(int i, int j, String cur, Trie node, boolean[][] visited, char[][] board, List<String> res){
        if(i<0||i>=board.length||j<0||j>=board[0].length||visited[i][j]==true)
            return;
        visited[i][j]=true;
        int idx=board[i][j]-'a';
        Trie nc=node.children[idx];
        if(nc!=null)
        {
            String str=cur+board[i][j];
            if(nc.isWord==true&&nc.added==false)
            {
                res.add(str);
                nc.added=true;
            }
            search(i-1,j,str,nc,visited,board,res);
            search(i+1,j,str,nc,visited,board,res);
            search(i,j-1,str,nc,visited,board,res);
            search(i,j+1,str,nc,visited,board,res);
        }
        visited[i][j]=false;
    }
}