# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def reverseEvenLengthGroups(self, head: Optional[ListNode]) -> Optional[ListNode]:
        def reverse(head, l):
            prev, cur, tail = None, head, head
            i = 0
            while cur and i < l:
                t = cur.next
                cur.next = prev
                prev = cur
                cur = t
                i += 1
            tail.next = cur
            return prev

        n = 0
        t = head
        while t:
            t = t.next
            n += 1
        dummy = ListNode(0, head)
        prev = dummy
        l = 1
        while (1 + l) * l // 2 <= n and prev:
            if l % 2 == 0:
                prev.next = reverse(prev.next, l)
            i = 0
            while i < l and prev:
                prev = prev.next
                i += 1
            l += 1
        left = n - l * (l - 1) // 2
        if left > 0 and left % 2 == 0:
            prev.next = reverse(prev.next, left)
        return dummy.next
