problems(swift): add “23. Merge k Sorted Lists”

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

problems/swift/merge-k-sorted-lists.swift

@@ -0,0 +1,41 @@
+/**
+ * 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 mergeKLists(_ lists: [ListNode?]) -> ListNode? {
+        var lists = lists.filter { $0 != nil }
+        let root = ListNode()
+
+        var node = root
+        while !lists.isEmpty {
+            // find the next list
+            var minIdx = 0
+            for i in 1..<lists.count {
+                if lists[i]!.val < lists[minIdx]!.val {
+                    minIdx = i
+                }
+            }
+
+            // insert the list into the node
+            node.next = lists[minIdx]
+            node = lists[minIdx]!
+
+            // shift the list at the index or remove it
+            if lists[minIdx]!.next == nil {
+                lists.remove(at: minIdx)
+            } else {
+                lists[minIdx] = lists[minIdx]!.next
+            }
+        }
+
+
+        return root.next
+    }
+}