public boolean exist(char[][] board, String word) {

char[] wordChar = word.toCharArray();

for(int i = 0; i < board.length; i++) {

for(int j = 0; j < board[i].length; j++) {

if(board[i][j] == word.charAt(0)) {

boolean result = search(board, wordChar, i, j, 0);

if(result) return true;

}

}

}

return false;

}

private boolean search(char[][] board, char[] word, int i, int j, int index) {

if(index == word.length) return true;

if(i < 0 || j < 0 || i == board.length || j == board[i].length) return false;

if(board[i][j] != word[index]) return false;

board[i][j] ^= 256; //相当于标记已遍历的字符，因为题目要求不能重复

boolean result = search(board, word, i+1, j, index+1)

|| search(board, word, i-1, j, index+1)

|| search(board, word, i, j+1, index+1)

|| search(board, word, i, j-1, index+1);

board[i][j] ^= 256;

return result;

}