문제https://leetcode.com/problems/swap-nodes-in-pairs 풀이재귀로 풀이next와 next.next를 고려해서 swap해야한다/** * Example: * var li = ListNode(5) * var v = li.`val` * Definition for singly-linked list. * class ListNode(var `val`: Int) { * var next: ListNode? = null * } */class Solution { fun swapPairs(head: ListNode?): ListNode? { if (head == null || head.next == null) return head; var second..