24. Swap Nodes In Pairs - Explanation

# 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: Optional[ListNode]) -> Optional[ListNode]:
        if not head:
            return None
        arr = []
        cur = head

        while cur:
            arr.append(cur)
            cur = cur.next

        for i in range(0, len(arr) - 1, 2):
            arr[i], arr[i + 1] = arr[i + 1], arr[i]

        for i in range(len(arr) - 1):
            arr[i].next = arr[i + 1]

        arr[-1].next = None
        return arr[0]

# 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: Optional[ListNode]) -> Optional[ListNode]:
        if not head or not head.next:
            return head

        cur = head
        nxt = head.next
        cur.next = self.swapPairs(nxt.next)
        nxt.next = cur
        return nxt

# 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: Optional[ListNode]) -> Optional[ListNode]:
        dummy = ListNode(0, head)
        prev, curr = dummy, head

        while curr and curr.next:
            nxtPair = curr.next.next
            second = curr.next

            # Reverse this pair
            second.next = curr
            curr.next = nxtPair
            prev.next = second

            # Update pointers
            prev = curr
            curr = nxtPair

        return dummy.next