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public class Solution {
    public boolean exist(char[][] board, String word) {
        if (board == null || board.length < 1 || board[0].length < 1) return false;
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if(dfs(board, i, j, word, 0)) {
                    return true;
                }
            }
        }
        return false;
    }
    private boolean dfs(char[][] board, int i, int j, String word, int start) {
        if (start == word.length()) {
            return true;
        }
        int row = board.length;
        int col = board[0].length;
        if (i < 0 || j < 0 || i >= row || j >= col || board[i][j] != word.charAt(start)) {
            return false;
        }
        
        board[i][j] ^= 256;
        boolean res =  dfs(board, i - 1, j, word, start + 1) || dfs(board, i, j - 1, word, start + 1) || dfs(board, i + 1, j, word, start + 1) || dfs(board, i, j + 1, word, start + 1) ;
        board[i][j] ^= 256;
        return res;
        
    }
}

public class Solution {
    public boolean exist(char[][] board, String word) {
        if (board == null || board.length == 0)
            return false;
        if (word == null || word.length() == 0)
            return true;

        char[] wordArray = word.toCharArray();
        int m = board.length;
        int n = board[0].length;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == wordArray[0] && existWord(board, i, j, wordArray, 1))
                    return true;
            }
        }
        return false;
    }

    private boolean existWord(char[][] board, int i, int j, char[] word, int index) {
        if (index == word.length)
            return true;

        char stored = board[i][j];
        board[i][j] = '*';
        boolean exist = i > 0 && board[i - 1][j] == word[index] && existWord(board, i - 1, j, word, index + 1)
                || i + 1 < board.length && board[i + 1][j] == word[index] && existWord(board, i + 1, j, word, index + 1)
                || j > 0 && board[i][j - 1] == word[index] && existWord(board, i, j - 1, word, index + 1)
                || j + 1 < board[0].length && board[i][j + 1] == word[index] && existWord(board, i, j + 1, word, index + 1);
        board[i][j] = stored;
        return exist;
    }
}