import copy


def _row2col(board):
    """Reflect across the "y=x" diagonal"""

    size = len(board)
    new_board = copy.deepcopy(board)

    for x in range(size):
        for y in range(x):
            new_board[y][x], new_board[x][y] = new_board[x][y], new_board[y][x]

    return new_board