feat: 21. Merge Two Sorted Lists solution

(cherry picked from commit 89322c3554301c0859b5e61e08d20f6ab995533e)

Author: Yiseull

merge-two-sorted-lists/Yiseull.java

@@ -0,0 +1,31 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        ListNode head = new ListNode(-1);
+        ListNode current = head;
+
+        while (list1 != null && list2 != null) {
+            if (list1.val <= list2.val) {
+                current.next = list1;
+                list1 = list1.next;
+            } else {
+                current.next = list2;
+                list2 = list2.next;
+            }
+            current = current.next;
+        }
+
+        current.next = (list1 != null) ? list1 : list2;
+
+        return head.next;
+    }
+}