Insertion sort of linked list

Time: O(N^2); Space: O(1); medium

Sort a linked list using insertion sort

The partial sorted list (black) initially contains only the first element in the list. With each iteration one element (red) is removed from the input data
and inserted in-place into the sorted list.

Example 1:

Input: linked-list: 4->2->1->3

Output: 1->2->3->4

Example 2:

Input: linked-list: -1->5->3->4->0

Output: -1->0->3->4->5

1. Insertion Sort

Algorithm

Insertion sort iterates, consuming one input element each repetition, and growing a sorted output list.

At each iteration, insertion sort removes one element from the input data, finds the location it belongs within the sorted list, and inserts it there.

It repeats until no input elements remain.

[1]:

class ListNode:
    """
    Singly-linked list
    """
    def __init__(self, x):
        self.val = x
        self.next = None

    def __repr__(self):
        if self:
            return "{} -> {}".format(self.val, repr(self.next))
        else:
            return None

class Solution1(object):
    def insertionSortList(self, head):
        '''
        :type head: ListNode
        :rtype: ListNode
        '''
        if head is None or self.isSorted(head):
            return head

        dummy = ListNode(-2147483648)
        dummy.next = head
        cur, sorted_tail = head.next, head

        while cur:
            prev = dummy
            while prev.next.val < cur.val:
                prev = prev.next
            if prev == sorted_tail:
                cur, sorted_tail = cur.next, cur
            else:
                cur.next, prev.next, sorted_tail.next = prev.next, cur, cur.next
                cur = sorted_tail.next

        return dummy.next

    def isSorted(self, head):
        while head and head.next:
            if head.val > head.next.val:
                return False
            head = head.next
        return True

[4]:

s = Solution1()
head = ListNode(4)
head.next = ListNode(2)
head.next.next = ListNode(1)
head.next.next.next = ListNode(3)
print(s.insertionSortList(head))

head = ListNode(-1)
head.next = ListNode(5)
head.next.next = ListNode(3)
head.next.next.next = ListNode(4)
head.next.next.next.next = ListNode(0)
print(s.insertionSortList(head))

1 -> 2 -> 3 -> 4 -> None
-1 -> 0 -> 3 -> 4 -> 5 -> None