# 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]:
dummy = ListNode(0)
dummy.next = head
prev = dummy
def reverseList(
head: Optional[ListNode],
) -> (Optional[ListNode], Optional[ListNode]):
prev, curr = None, head
while curr:
nxt = curr.next
curr.next = prev
prev = curr
curr = nxt
return (prev, head)
while True:
count = 0
start = prev.next
end = prev
for i in range(k):
end = end.next
if end is None:
return dummy.next
next_group_head = end.next
end.next = None
reversed_head, reversed_tail = reverseList(start)
prev.next = reversed_head
reversed_tail.next = next_group_head
prev = reversed_tail
return dummy.next