Word Search

public class Solution {    public boolean exist(char[][] board, String word) {        if (board == null || board.length == 0 || board[0].length == 0) {            return false;        }        int m = board.length, n = board[0].length;        boolean[][] visited = new boolean[m][n];        for (int i = 0; i < m; i++) {            for (int j = 0; j < n; j++) {                if (helper(board, visited, word, 0, i, j)) {                    return true;                }            }        }        return false;    }        private boolean helper(char[][] board, boolean[][] visited, String word, int pos, int i, int j) {        if (pos == word.length()) {            return false;        }        if (i < 0 || i == board.length || j < 0 || j == board[0].length) {            return false;        }        if (visited[i][j] || board[i][j] != word.charAt(pos)) {            return false;        }        if (pos + 1 == word.length()) {            return true;        }        visited[i][j] = true;        boolean flag = helper(board, visited, word, pos + 1, i + 1, j) || helper(board, visited, word, pos + 1, i - 1, j) || helper(board, visited, word, pos + 1, i, j + 1) || helper(board, visited, word, pos + 1, i, j - 1);        visited[i][j] = false;        return flag;    }}