我正在尝试用Python解出一个6x6数独

def solve_sudoku(board):
    if is_complete(board):
        return board

    row, col = find_empty_cell(board)
    for num in range(1, 7):
        if is_valid(board, row, col, num):
            board[row][col] = num
            if solve_sudoku(board):
                return board
            board[row][col] = 0

    return None

def is_complete(board):
    for row in range(6):
        for col in range(6):
            if board[row][col] == 0:
                return False
    return True

def find_empty_cell(board):
    for row in range(6):
        for col in range(6):
            if board[row][col] == 0:
                return row, col
    return None

def is_valid(board, row, col, num):
    # Check if num already exists in the row
    for i in range(6):
        if board[row][i] == num:
            return False

    # Check if num already exists in the column
    for i in range(6):
        if board[i][col] == num:
            return False

    # Check if num already exists in the 2x3 sub-grid
    start_row = (row // 2) * 2
    start_col = (col // 3) * 3
    for i in range(start_row, start_row + 2):
        for j in range(start_col, start_col + 3):
            if board[i][j] == num:
                return False

    return True

# Example 6x6 Sudoku board
board = [
    [0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0],
    [0, 0, 0, 0, 0, 0]
]

solution = solve_sudoku(board)
if solution:
    print("Solution:")
    for row in solution:
        print(row)
else:
    print("No solution exists.")