82. remove duplicates from sorted list ii

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.

solution

Iterate through the whole block before deciding to add or remove the value (use a helper function).

def deleteDuplicates(self, head: Optional[ListNode]):
	dummy = ListNode()
	prev, cur = dummy, head
	  
	def traverse_block(node):
		prev = node.val
		length = 0
		while node and node.val == prev:
			node = node.next
			length += 1
		return length, node
	  
	while cur:
		length, nex = traverse_block(cur)
		# take the value
		if length == 1:
			prev.next = cur
			prev = cur
			cur = cur.next
		else: # skip the block
			cur = nex
			prev.next = None
	  
	return dummy.next