79. 单词搜索

board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]

给定 word = "ABCCED", 返回 true.
给定 word = "SEE", 返回 true.
给定 word = "ABCB", 返回 false.

public boolean exist(char[][] board, String word) {
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (exist(board, i, j, word, 0)) {
                    return true;
                }
            }
        }
        return false;
    }

    private boolean exist(char[][] board, int i, int j, String word, int start) {
        if (start >= word.length()) {
            return true;
        }
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length) {
            return false;
        }

        if (board[i][j] == word.charAt(start++)) {
            char c = board[i][j];
            board[i][j] = '#';
            boolean res =
                    exist(board, i + 1, j, word, start) ||
                            exist(board, i - 1, j, word, start) ||
                            exist(board, i, j + 1, word, start) ||
                            exist(board, i, j - 1, word, start);
            board[i][j] = c;
            return res;
        }
        return false;
    }