leetcode-24-两两交换链表中的节点

class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        dummy=ListNode(0)
        p = dummy
        h = head
        while h:
            if h and h.next:
                p.next = ListNode(h.next.val)
                p = p.next
                p.next = ListNode(h.val)
                h = h.next.next
            else:
                p.next = ListNode(h.val)
            p = p.next
        return dummy.next

class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        dummy=ListNode(0)
        p = dummy
        h = head
        while h:
            if h and h.next:
                tmp = h.next
                p.next = tmp
                h.next = h.next.next
                tmp.next = h
                h = h.next
                p = p.next.next
            else:
                p.next = h
                h = h.next
        return dummy.next

class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        if not head or not head.next:
            return head
        tmp = head.next 
        head.next = self.swapPairs(head.next.next) 
        tmp.next = head 
        return tmp