kt: add «21. Merge Two Sorted Lists»

URL:	https://leetcode.com/problems/merge-two-sorted-lists/
Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/merge-two-sorted-lists.kt

@@ -0,0 +1,31 @@
+class Solution {
+    fun mergeTwoLists(
+        list1: ListNode?,
+        list2: ListNode?,
+    ): ListNode? {
+        val head = ListNode(0)
+        var (l1, l2) = list1 to list2
+
+        var node = head
+        while (l1 != null && l2 != null) {
+            if (l1!!.`val` < l2!!.`val`) {
+                node.next = l1
+                l1 = l1!!.next
+            } else {
+                node.next = l2
+                l2 = l2!!.next
+            }
+
+            node = node.next
+        }
+
+        l1?.let {
+            node.next = it
+        }
+        l2?.let {
+            node.next = it
+        }
+
+        return head.next
+    }
+}