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.

class Solution {
    int[][] directions = {{1, 0}, {0, 1}, {-1, 0}, {0, -1}};
    boolean[][] mark = null;

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

        if (word == null || board.length == 0) {
            return false;
        }
        mark = new boolean[board.length][board[0].length];
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (board[i][j] == word.charAt(0)) {
                    mark[i][j] = true;
                    if (exist(board, word, 1, i, j)) {
                        return true;
                    }
                    mark[i][j] = false;
                }
            }
        }
        return false;
    }

    private boolean exist(char[][] board, String word, int index, int i, int j) {
        if (word.length() == index) {
            return true;
        }
        for (int temp = 0; temp < directions.length; temp++) {
            int newI = i + directions[temp][0];
            int newJ = j + directions[temp][1];
            if (newI >= 0 && newI < board.length && newJ >= 0 && newJ < board[0].length) {
                if (!mark[newI][newJ] && word.charAt(index) == board[newI][newJ]) {
                    mark[newI][newJ] = true;
                    if (exist(board, word, index + 1, newI, newJ)) {
                        return true;
                    }
                    mark[newI][newJ] = false;
                }
            }
        }
        return false;
    }
}

深搜
用一个二维数组表示上下左右四个方向
另一个二维数组表示是否访问过该字母