problems(swift): add “21. Merge Two Sorted Lists”

Signed-off-by: Matej Focko <mfocko@redhat.com>

swift/merge-two-sorted-lists.swift

@@ -0,0 +1,42 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init() { self.val = 0; self.next = nil; }
+ *     public init(_ val: Int) { self.val = val; self.next = nil; }
+ *     public init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next; }
+ * }
+ */
+class Solution {
+    func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
+        var list1 = list1
+        var list2 = list2
+
+        let head = ListNode()
+
+        var node: ListNode? = head
+        while list1 != nil && list2 != nil {
+            if list1!.val < list2!.val {
+                node!.next = list1
+                node = node!.next
+
+                list1 = list1!.next
+            } else {
+                node!.next = list2
+                node = node!.next
+
+                list2 = list2!.next
+            }
+        }
+
+        if list1 != nil {
+            node!.next = list1
+        }
+        if list2 != nil {
+            node!.next = list2
+        }
+
+        return head.next
+    }
+}