[Problem] Remove Duplicates from Sorted List II

Problem

Given the head of a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list. Return the linked list sorted as well.

Example

Input: head = [1,2,3,3,4,4,5]
Output: [1,2,5]

Approach

1. Foremost, we remove the head in case it has an duplicate (For example, [1, 1, 1, 2, 3, 4]).
2. We remove the internal duplicates inside the list.

For this process, I first check whether the head.val and head.next.val are equivalent, determining if step number 1 must be processed. Here, I simply reallocated the head to the nodes coming after the duplicates.

During the second step, I created two nodes prev and curr, each keeping track of duplicates. In the case of a duplicate (checked through curr), I followed a similar process where I take out the entire section covered by curr.

Solution Code

def deleteDuplicates(self, head):
    if head == None: return head
    if head.next == None: return head
    
    ## duplicate -> remove
    while head and head.next != None and head.val == head.next.val:
        while head and head.next != None and head.val == head.next.val:
            head = head.next
        head = head.next

    if head == None: return head
    if head.next == None: return head

    prev = head
    curr = head.next

    while curr:
        #print(head)
        if curr.next and curr.val == curr.next.val:

            while curr.next and curr.val == curr.next.val:
                curr = curr.next
            prev.next = curr.next
            curr = prev.next

        else:
            prev = prev.next
            curr = curr.next

    return head