NO22.两两交换链表中的节点

#迭代
#1.point.next = swap2
#2.swap1.next = swap2.next
#3.swap2.next = swap1
# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        dummy = ListNode(-1)#设置蓄力节点，此时为-1或者0都不影响
        dummy.next = head
        point = dummy
        while (point.next  and point.next.next) :#如图所示
            swap1 = point.next
            swap2 = point.next.next
            point.next = swap2
            swap1.next = swap2.next
            swap2.next = swap1
            point = swap1
        return dummy.next

#递归
#由于每次都是俩俩交换，很有规律性
#递归真属于一看就会，一做就废
class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        if head == None or head.next == None:
            return head
        l1 = head.next
        head.next = self.swapPairs(head.next.next)
        l1.next = head
        return l1