1
class Solution:
2
    def reverse(self, head: Optional[ListNode]) -> Optional[ListNode]:
3
        cur = head
4
        pre = None
5

6
        while cur:
7
            nxt = cur.next
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            cur.next = pre
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            pre = cur
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            cur = nxt
11

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        return pre
13

14
    def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
15
        dummy = ListNode(0, head)
16

17
        cur = dummy.next
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        cur_head = cur
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        pre_cur_head = dummy
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        cnt = 0
21

22
        while cur:
23
            cnt += 1
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            nxt = cur.next
25
            if cnt >= 2:
26
                cur.next = None
27
                new_head = self.reverse(cur_head)
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                cur_head.next = nxt
29
                pre_cur_head.next = new_head
30

31
                pre_cur_head = cur_head
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                cur_head = nxt
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                cnt = 0
34

35
            cur = nxt
36

37
        return dummy.next

1
class Solution:
2
    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
3
        left = right = dummy = ListNode(0, head)
4

5
        for _ in range(n):
6
            right = right.next
7
        while right.next:
8
            left = left.next
9
            right = right.next
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        left.next = left.next.next
11

12
        return dummy.next