79. 单词搜索

给定一个二维网格和一个单词，找出该单词是否存在于网格中。

单词必须按照字母顺序，通过相邻的单元格内的字母构成，其中“相邻”单元格是那些水平相邻或垂直相邻的单元格。同一个单元格内的字母不允许被重复使用。

示例:

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.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/word-search
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

public class Solution {
    private boolean[][] flag = null;
    private char[] words = null;
    private int row = 0;
    private int col = 0;

    // i,j是word[0]的位置; cur表示words中的位置
    private boolean helper(char[][] board, int cur, int i, int j) {
        if (cur == words.length)
            return true;
        // 上
        if (i-1 >= 0 && !flag[i-1][j] && board[i-1][j] == words[cur]) {
            flag[i-1][j] = true;
            if (helper(board, cur+1, i-1,j))
                return true;
            flag[i-1][j] = false;
        }
        // 下
        if (i+1 < row && !flag[i+1][j] && board[i+1][j] == words[cur]) {
            flag[i+1][j] = true;
            if (helper(board, cur+1, i+1, j))
                return true;
            flag[i+1][j] = false;
        }
        // 左
        if (j-1 >= 0 && !flag[i][j-1] && board[i][j-1] == words[cur]) {
            flag[i][j-1] = true;
            if (helper(board, cur+1, i, j-1))
                return true;
            flag[i][j-1] = false;
        }
        // 右
        if (j+1 < col && !flag[i][j+1] && board[i][j+1] == words[cur]) {
            flag[i][j+1] = true;
            if (helper(board, cur+1, i, j+1))
                return true;
            flag[i][j+1] = false;
        }
        return false;
    }

    public boolean exist(char[][] board, String word) {
        words = word.toCharArray();
        flag = new boolean[board.length][board[0].length];
        row = board.length;
        col = board[0].length;
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (board[i][j] == words[0]) {
                    flag[i][j] = true;
                    if (helper(board, 1, i, j))
                        return true;
                    flag[i][j] = false;
                }
            }
        }
        return false;
    }

    public static void main(String[] args) {
        char[][] board = {
                {'A','B','C','E'},
                {'S','F','C','S'},
                {'A','D','E','E'}
        };
        boolean abcced = new Solution().exist(board, "SEE");
        System.out.println("abcced = " + abcced);
    }
}