Решение

class Solution {
    fun mergeTwoLists(list1: ListNode?, list2: ListNode?): ListNode? {
        var a = list1
        var b = list2
        val dummy = ListNode(0)
        var t = dummy
        while (a != null && b != null) {
            if (a.`val` <= b.`val`) {
                t.next = a
                a = a.next
            } else {
                t.next = b
                b = b.next
            }
            t = t.next!!
        }
        t.next = a ?: b
        return dummy.next
    }
}