Insertion Sort with a Linked List

Question

I am trying to create an insertion sort with linked lists. Here is what I have:

def insertion_sort(a):
        """
        -------------------------------------------------------
        Sorts a list using the Insertion Sort algorithm.
        Use: insertion_sort( a )
        -------------------------------------------------------
        Preconditions:
          a - linked list of comparable elements (?)
        Postconditions:
          Contents of a are sorted.
        -------------------------------------------------------
        """        
        unsorted = a._front
        a._front = None

        while unsorted is not None and unsorted._next is not None:
            current = unsorted
            unsorted = unsorted._next

            if current._value < unsorted._value:
                current._next = unsorted._next
                unsorted._next = current
                unsorted = unsorted._next
            else:
                find = unsorted
                while find._next is not None and current._value > find._next._value:
                    find = find._next

                current._next = find._next
                current = find._next
            a._front = unsorted

        return a

I believe what I have is correct in terms of sorting. However when I try to read the list in the main module I get a bunch of None values.

In this case, the insertion sort is not creating a new list when sorting. Rather, it is moving all sorted elements to the 'front'.

To summarize, I have two problems: I am not sure if the insertion sort is correct, and there are problems with the returned list a as it contains None values. Thanks in advance

AChampion · Accepted Answer · 2015-04-03 00:37:32Z

2

Not exactly sure about the type of a is, but if you assume a simple:

class Node:
    def __init__(self, value, node=None):
        self._value = value
        self._next = node
    def __str__(self):
        return "Node({}, {})".format(self._value, self._next)

Then your insertion sort isn't far off, it needs to handle the head case properly:

def insertion_sort(unsorted):    
    head = None
    while unsorted:
        current = unsorted
        unsorted = unsorted._next
        if not head or current._value < head._value:
            current._next = head;
            head = current;
        else:
            find = head;
            while find and current._value > find._next._value:
                find = find._next
            current._next = find._next
            find._next = current
    return head

>>> print(insertion_sort(Node(4, Node(1, Node(3, Node(2))))))
Node(1, Node(2, Node(3, Node(4, None))))

answered Apr 3, 2015 at 0:37

AChampion

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2 Comments

user3170251 Over a year ago

a is a linked list passed on from the main module. Does this change anything

AChampion Over a year ago

if a has a similar structure to above then should be fine, the algorithm works so you just need to fit your structure to it.

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