编写一段python代码，使其利用DFS算法解决八数码问题

在Python中，我们可以使用深度优先搜索（Depth First Search，简称DFS）来解决经典的八数码问题（也称为数独）。以下是使用递归实现DFS的一个简单示例：

def is_valid(board, row, col, num):
    # 检查行
    for i in range(9):
        if board[row][i] == num:
            return False

    # 检查列
    for i in range(9):
        if board[i][col] == num:
            return False

    # 获取当前九宫格的左上角坐标
    box_x = (row // 3) * 3
    box_y = (col // 3) * 3

    # 检查九宫格
    for i in range(box_x, box_x + 3):
        for j in range(box_y, box_y + 3):
            if board[i][j] == num:
                return False

    return True


def solve_sudoku(board):
    def dfs(row, col):
        nonlocal solved
        if row == 9:
            solved = True
            return True

        for num in range(1, 10):
            if is_valid(board, row, col, num):
                board[row][col] = num
                
                # 继续处理下一行或列的下一个数字
                if dfs(row + 1, col) or dfs(row, col + 1):
                    return True
                    
                # 如果回溯到这里，说明尝试这个位置失败了，恢复并继续寻找其他可能
                board[row][col] = 0
        return False

    solved = False
    dfs(0, 0)
    return solved, board if solved else None


# 示例：解决一个数独谜题
board = [
    [5, 3, 0, 0, 7, 0, 0, 0, 0],
    [6, 0, 0, 1, 9, 5, 0, 0, 0],
    [0, 9, 8, 0, 0, 0, 0, 6, 0],
    [8, 0, 0, 0, 6, 0, 0, 0, 3],
    [4, 0, 0, 8, 0, 3, 0, 0, 1],
    [7, 0, 0, 0, 2, 0, 0, 0, 6],
    [0, 6, 0, 0, 0, 0, 2, 8, 0],
    [0, 0, 0, 4, 1, 9, 0, 0, 5],
    [0, 0, 0, 0, 8, 0, 0, 7, 9]
]

solved, result_board = solve_sudoku(board)
if solved:
    print("Sudoku solved!")
else:
    print("No solution exists.")