feat: Add solution for LeetCode problem 21

Author: WhiteHyun

merge-two-sorted-lists/WhiteHyun.swift

@@ -0,0 +1,34 @@
+//
+//  21. Merge Two Sorted Lists.swift
+//  https://leetcode.com/problems/merge-two-sorted-lists/description/
+//  Algorithm
+//
+//  Created by 홍승현 on 2024/05/04.
+//
+
+import Foundation
+
+final class LeetCode21 {
+  func mergeTwoLists(_ list1: ListNode?, _ list2: ListNode?) -> ListNode? {
+    let dummy: ListNode? = .init(0)
+    var currentNode: ListNode? = dummy
+    var l1 = list1
+    var l2 = list2
+
+    while l1 != nil, l2 != nil {
+      if l1!.val < l2!.val {
+        currentNode?.next = l1
+        l1 = l1?.next
+      } else {
+        currentNode?.next = l2
+        l2 = l2?.next
+      }
+
+      currentNode = currentNode?.next
+    }
+
+    currentNode?.next = l1 ?? l2
+
+    return dummy?.next
+  }
+}