feat: add swift implementation to lcof2 problem: No.078 (doocs#3457)

Author: klever34

lcof2/剑指 Offer II 078. 合并排序链表/README.md

@@ -363,6 +363,54 @@ def merge_two_lists(l1, l2)
 end
 ```
 
+#### Swift
+
+```swift
+/** class ListNode {
+*    var val: Int
+*    var next: ListNode?
+*    init() { self.val = 0; self.next = nil }
+*    init(_ val: Int) { self.val = val; self.next = nil }
+*    init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next }
+* }
+*/
+
+class Solution {
+    func mergeKLists(_ lists: [ListNode?]) -> ListNode? {
+        let n = lists.count
+        if n == 0 {
+            return nil
+        }
+        
+        var mergedList: ListNode? = lists[0]
+        for i in 1..<n {
+            mergedList = mergeLists(mergedList, lists[i])
+        }
+        return mergedList
+    }
+    
+    private func mergeLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
+        let dummy = ListNode()
+        var cur = dummy
+        var l1 = l1
+        var l2 = l2
+        
+        while let node1 = l1, let node2 = l2 {
+            if node1.val <= node2.val {
+                cur.next = node1
+                l1 = node1.next
+            } else {
+                cur.next = node2
+                l2 = node2.next
+            }
+            cur = cur.next!
+        }
+        cur.next = l1 ?? l2
+        return dummy.next
+    }
+}
+```
+
 <!-- tabs:end -->
 
 <!-- solution:end -->

lcof2/剑指 Offer II 078. 合并排序链表/Solution.swift

@@ -0,0 +1,43 @@
+/** class ListNode {
+*    var val: Int
+*    var next: ListNode?
+*    init() { self.val = 0; self.next = nil }
+*    init(_ val: Int) { self.val = val; self.next = nil }
+*    init(_ val: Int, _ next: ListNode?) { self.val = val; self.next = next }
+* }
+*/
+
+class Solution {
+    func mergeKLists(_ lists: [ListNode?]) -> ListNode? {
+        let n = lists.count
+        if n == 0 {
+            return nil
+        }
+        
+        var mergedList: ListNode? = lists[0]
+        for i in 1..<n {
+            mergedList = mergeLists(mergedList, lists[i])
+        }
+        return mergedList
+    }
+    
+    private func mergeLists(_ l1: ListNode?, _ l2: ListNode?) -> ListNode? {
+        let dummy = ListNode()
+        var cur = dummy
+        var l1 = l1
+        var l2 = l2
+        
+        while let node1 = l1, let node2 = l2 {
+            if node1.val <= node2.val {
+                cur.next = node1
+                l1 = node1.next
+            } else {
+                cur.next = node2
+                l2 = node2.next
+            }
+            cur = cur.next!
+        }
+        cur.next = l1 ?? l2
+        return dummy.next
+    }
+}