24.py

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next

# Swap the value not the node
class Solution:
    def swapPairs(self, head: ListNode) -> ListNode:
        node = head

        while node and node.next:
            node.val, node.next.val = node.next.val, node.val
            node = node.next.next

        return head

# Recursive


class Solution2:
    def swapPairs(self, head: ListNode) -> ListNode:

        if not head or not head.next:
            return head

        first_node = head
        second_node = head.next

        first_node.next = self.swapPairs(second_node.next)
        second_node.next = first_node

        return second_node

# Iterative
class Solution3:
    def swapPairs(self, head):
        pre, pre.next = self, head
        while pre.next and pre.next.next:
            a = pre.next
            b = a.next
            pre.next, b.next, a.next = b, a, b.next
            pre = a
        return self.next