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# iteration
def reverseN(head: listNode, n: int) -> listNode:
    if head is None or head.next is None:
        return head
    pre, cur, nxt = None, head, head.next
    while n and cur:
        cur.next = pre
        pre = cur
        cur = nxt
        if nxt is not None:
            nxt = nxt.next
        n -= 1
    head.next = cur
    return pre

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# recursion
successor = None
def reverseN(head: ListNode, n: int):
    global successor
    if n == 1:
        successor = head.next
        return head
    last = reverseN(head.next, n-1)
    head.next.next = head
    head.next = successor
    return last