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# Runtime: 3864 ms (Top 51.07%) | Memory: 53.5 MB (Top 68.62%)
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class Solution:
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    def reverseEvenLengthGroups(self, head: Optional[ListNode]) -> Optional[ListNode]:
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        group = 2
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        tail = head  # tail of previous group
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        while tail and tail.next:
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            cnt = 1  # actual size of the current group
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            cur = tail.next  # first node of the current group
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            while cur.next and cnt < group:
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                cur = cur.next
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                cnt += 1
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            pre, cur = tail, tail.next
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            if cnt % 2 == 0:  # if group size is even
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                while cnt and cur:
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                    nxt = cur.next
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                    cur.next = pre
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                    pre = cur
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                    cur = nxt
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                    cnt -= 1
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                first = tail.next  # first node of the original group
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                first.next = cur
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                tail.next = pre
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                tail = first
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            else:
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                while cnt and cur:
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                    pre, cur = cur, cur.next
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                    cnt -= 1
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                tail = pre
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            group += 1
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        return head