I am trying to sort a linked list in descending order
<p>Can you please help me, i am trying to sort the linked lists according to the documentation in the add function but i am getting an error which says:
f.add('b', 2)
File "", line 69, in add
AttributeError: 'NoneType' object has no attribute 'next'
how can i avoid this?
Thankyou.</p>

<pre><code>class Frequency(object):
    """

    Stores a letter:frequency pair.

    &gt;&gt;&gt; f = Frequency('c', 2)
    &gt;&gt;&gt; f.letter
    'c'
    &gt;&gt;&gt; f.frequency
    2
    &gt;&gt;&gt; f
    {c: 2}
    """
    def __init__(self, letter, frequency):
        self.letter = letter
        self.frequency = frequency
        self.next = None

    def __repr__(self):
        return '{%s: %d}' % (self.letter, self.frequency)

class SortedFrequencyList(object):
    """
    Stores a collection of Frequency objects as a sorted linked list.
    Items are sorted from the highest frequency to the lowest.
    """
    def __init__(self):
        self.head = None

    def add(self, letter, frequency):
        """
        Adds the given `letter`:`frequency` combination as a Frequency object
        to the list. If the given `letter` is already in the list, the given
        `frequency` is added to its frequency.

        &gt;&gt;&gt; f = SortedFrequencyList()
        &gt;&gt;&gt; f.add('a', 3)
        &gt;&gt;&gt; f
        ({a: 3})
        &gt;&gt;&gt; f.add('b', 2)
        &gt;&gt;&gt; f
            ({a: 3}, {b: 2})
        &gt;&gt;&gt; f.add('c', 4)
        &gt;&gt;&gt; f
        ({c: 4}, {a: 3}, {b: 2})
        &gt;&gt;&gt; f.add('b', 3)
        &gt;&gt;&gt; f
        ({b: 5}, {c: 4}, {a: 3})
        """

        current = self.head
        found = False
        if self.head is None:
            self.head = Frequency(letter, frequency)
        else:
            prev = None
            while current is not None:
                if current.letter == letter:
                    current.frequency = current.frequency + frequency
                    found = True
                prev = current
                current = current.next

                next1 = current.next
                if next1 is None:
                    current = next1

                if current.frequency &lt; next1.frequency:
                    temp = current
                    current = next1
                    next1 = temp
                else:
                    current = next1
                    next1 = current.next.next


            if found is False:
                prev.next = Frequency(letter, frequency)
</code></pre>
