Solution.cs

public class Solution {
    private int m;
    private int n;
    private char[][] board;
    private string word;

    public bool Exist(char[][] board, string word) {
        m = board.Length;
        n = board[0].Length;
        this.board = board;
        this.word = word;
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (dfs(i, j, 0)) {
                    return true;
                }
            }
        }
        return false;
    }

    private bool dfs(int i, int j, int k) {
        if (k == word.Length - 1) {
            return board[i][j] == word[k];
        }
        if (board[i][j] != word[k]) {
            return false;
        }
        char c = board[i][j];
        board[i][j] = '0';
        int[] dirs = { -1, 0, 1, 0, -1 };
        for (int u = 0; u < 4; ++u) {
            int x = i + dirs[u];
            int y = j + dirs[u + 1];
            if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] != '0' && dfs(x, y, k + 1)) {
                return true;
            }
        }
        board[i][j] = c;
        return false;
    }
}