Merge pull request #2929 from drxlx/0023-merge-k-sorted-lists

Create 0023-merge-k-sorted-lists.swift

Author: felivalencia3

swift/0023-merge-k-sorted-lists.swift

@@ -0,0 +1,62 @@
+/**
+ * Question Link: https://leetcode.com/problems/merge-k-sorted-lists/
+ */
+
+ /**
+ * 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 MergeKLists {
+    func mergeKLists(_ lists: [ListNode?]) -> ListNode? {
+        guard lists.contains(where: { $0 != nil }) else { return nil }
+
+        var lists = lists
+
+        while lists.count > 1 {
+            var mergedLists = [ListNode?]()
+
+            for i in stride(from: 0, to: lists.count, by: 2) {
+                let l1 = lists[i]
+                let l2 = (i + 1 < lists.count) ? lists[i + 1] : nil
+                mergedLists.append(self.merge(l1: l1, l2: l2))
+            }
+            lists = mergedLists
+        }
+
+        return lists.first!
+    }
+
+    func merge(l1: ListNode?, l2: ListNode?) -> ListNode? {
+        var dummy = ListNode()
+        var tail: ListNode? = dummy 
+        var l1 = l1
+        var l2 = l2
+
+        while l1 != nil && l2 != nil {
+            if l1!.val < l2!.val {
+                tail?.next = l1
+                l1 = l1?.next
+            } else {
+                tail?.next = l2!
+                l2 = l2?.next
+            }
+            tail = tail?.next
+        }
+
+        if l1 != nil {
+            tail?.next = l1
+        }
+        if l2 != nil {
+            tail?.next = l2
+        }
+
+        return dummy.next
+    }
+}