[LinkedList] add solution to MergeKSortedLists

LinkedList/MergeKSortedLists.swift

@@ -0,0 +1,68 @@
+/**
+ * Question Link: https://leetcode.com/problems/merge-k-sorted-lists/
+ * Primary idea: Divide and Conqure with merge two sorted lists as a helper function
+ * Time Complexity: O(mlogn), m stands for the length of one list, n stands for the number of lists
+ *                  Space Complexity: O(1)
+ *
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     public var val: Int
+ *     public var next: ListNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.next = nil
+ *     }
+ * }
+ */
+
+class MergeKSortedLists {
+    func mergeKLists(lists: [ListNode?]) -> ListNode? {
+        guard lists.count > 0 else {
+            return nil
+        }
+
+        var left = 0
+        var right = lists.count - 1
+
+        var lists = lists
+
+        while right > 0 {
+            left = 0
+            while left < right {
+                lists[left] = _mergeTwoLists(lists[left], lists[right])
+                left += 1
+                right -= 1
+            }
+        }
+
+        return lists[0]
+    }
+
+    private func _mergeTwoLists(l1: ListNode?, _ l2: ListNode?) -> ListNode? {
+        let dummy = ListNode(0)
+        var node = dummy
+
+        var l1 = l1
+        var l2 = l2
+
+        while l1 != nil && l2 != nil {
+            if l1!.val < l2!.val {
+                node.next = l1
+                l1 = l1!.next
+            } else {
+                node.next = l2
+                l2 = l2!.next
+            }
+
+            node = node.next!
+        }
+
+        if l1 != nil {
+            node.next = l1
+        } else {
+            node.next = l2
+        }
+
+        return dummy.next
+    }
+}