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import java.util.ArrayList;
import java.util.List;

class Solution {
    static boolean flag = false;
    static boolean[][] isFlag;

    public boolean exist(char[][] board, String word) {
        flag = false; // 每次调用要重置
        isFlag = new boolean[board.length][board[0].length];

        List<Character> list = new ArrayList<>();
        for (int i = 0; i < word.length(); i++) {
            list.add(word.charAt(i));
        }

        // 从每一个点开始尝试匹配
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                dfs(i, j, board, list, 0); // 第0个字符开始匹配
                if (flag) return true;
            }
        }
        return false;
    }

    public static void dfs(int i, int j, char[][] board, List<Character> list, int index) {
        // 如果已经全部匹配完了
        if (index == list.size()) {
            flag = true;
            return;
        }

        // 越界 + 已访问 + 当前字符不匹配
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length ||
                isFlag[i][j] || board[i][j] != list.get(index)) {
            return;
        }

        isFlag[i][j] = true;

        // 递归向四个方向搜索
        dfs(i + 1, j, board, list, index + 1);
        dfs(i - 1, j, board, list, index + 1);
        dfs(i, j + 1, board, list, index + 1);
        dfs(i, j - 1, board, list, index + 1);

        isFlag[i][j] = false; // 回溯
    }
}