24. Swap Nodes in Pairs

https://leetcode.com/problems/swap-nodes-in-pairs

Python

• First Try

class Solution:
    def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head:
            return

        cur = head
        dummy_head = ListNode(val=None, next=head)
        pre = dummy_head
        while cur and cur.next:
            next_cur = cur.next.next
            new_left = self._swap(cur, cur.next)
            pre.next = new_left
            pre = new_left.next
            cur = next_cur

        return dummy_head.next

    def _swap(self, left, right):
        if not right:
            return left
        left.next, right.next = right.next, left
        return right

• Second Try, more clear

class Solution:
    def swapPairs(self, head: Optional[ListNode]) -> Optional[ListNode]:
        if not head or not head.next:
            return head

        left = head
        right = head.next

        left.next = self.swapPairs(right.next)
        right.next = left

        return right

Go

func swapPairs(head *ListNode) *ListNode {
    if (head == nil || head.Next == nil) {
        return head
    }

    left := head
    right := head.Next

    left.Next = swapPairs(right.Next)
    right.Next = left

    return right
}