单词搜索

/*
题目思路：
1. 使用DFS（深度优先搜索）和Java Stream API来处理二维网格和字符串的匹配问题。
2. 遍历二维网格的每个单元格，使用Stream和filter方法筛选出符合条件的起始点，进行递归搜索。
3. 使用Streams的任何操作都必须避免副作用，标记和回溯需要考虑副作用问题。
*/
 
import java.util.stream.IntStream;
 
public class Solution {
    public boolean exist(char[][] board, String word) {
        return IntStream.range(0, board.length)
            .boxed()
            .flatMap(i -> IntStream.range(0, board[0].length)
                    .filter(j -> dfs(board, word, i, j, 0))
                    .boxed())
            .findFirst()
            .isPresent();
    }
 
    private boolean dfs(char[][] board, String word, int i, int j, int index) {
        if (index == word.length()) return true;
        if (i < 0 || j < 0 || i >= board.length || j >= board[0].length || board[i][j] != word.charAt(index))
            return false;
        char temp = board[i][j];
        board[i][j] = ' ';
        boolean found = dfs(board, word, i + 1, j, index + 1)
                || dfs(board, word, i - 1, j, index + 1)
                || dfs(board, word, i, j + 1, index + 1)
                || dfs(board, word, i, j - 1, index + 1);
        board[i][j] = temp;
        return found;
    }
}