Problem URL: Remove duplicates from sorted list II

We will have 2 pointers, when we iterate over each node, we check if we found duplicate, then we move right pointer until we hit another value. Then remove all nodes between 2 pointers and move both pointers.

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

class Solution:
    def deleteDuplicates(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head or not head.next:
            return head

        dummy = ListNode(-1, head)
        prev = dummy

        while head:
            while head.next and head.next.val == head.val:
                head = head.next
            if prev.next == head:
                prev = head
            else:
                prev.next = head.next
            head = head.next

        return dummy.next

Time Complexity: O(n)
Space Complexity: O(1)