I had difficulties with this problem with terminating completed lists. I was surprised that ChatGPT didn’t give this simple solution, it suggested a large rework. I’ll have to be mindful before implementing all of ChatGPT’s suggestions.

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Solution

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseKGroup(self, head: Optional[ListNode], k: int) -> Optional[ListNode]:
        batch = []
        start = None
        curr = None
        while head:
            batch.append(head)
            head = head.next
            if len(batch) == k:
                if curr:
                    curr.next = batch.pop()
                    curr = curr.next
                else:
                    curr = batch.pop()
                    if not start:
                        start = curr
                     
                while batch:
                    curr.next = batch.pop()
                    curr = curr.next
                
        if batch and curr:
            curr.next = batch[0]
        elif batch:
            start = batch[0]
        elif not batch and curr:
            curr.next = None
        
        return start

Time complexity is O(n) = n and space complexity is O(k).