leetcode 79 word search

class  {
    public boolean exist(char[][] board, String word) {
        if(board == null || board.length == 0) return false;
        int m = board.length;
        int n = board[0].length;
        for(int i=0; i<m; i++){
            for(int j=0; j<n; j++){
                if(search(board, word, i, j, 0)){
                    return true;
                }
            }
        }
        return false;
    }
    public boolean search(char[][] board, String word, int i, int j, int pos){
        if(i<0 || j < 0 || i >= board.length || j >= board[0].length)
            return false;
        if(board[i][j] != word.charAt(pos)) return false;
        if(board[i][j] == word.charAt(pos) && pos == word.length()-1)
            return true;
        board[i][j] = ' ';
        boolean result = 
            search(board, word, i-1, j, pos+1) ||
            search(board, word, i+1, j, pos + 1) ||
            search(board, word, i, j-1, pos + 1) ||
            search(board, word, i, j+1, pos + 1);
        board[i][j] = word.charAt(pos);
        return result;
    }
}