Wrote a shorter version of the two number adder.

P002-Add-Two-Numbers/P002-Add-Two-Numbers/Program.cs

@@ -23,7 +23,12 @@ namespace P002_Add_Two_Numbers
             input2.next = new ListNode(6);
             input2.next.next = new ListNode(4);
 
-            var answer = solution.AddTwoNumbers(input1, input2);
+            var answer = solution.AddTwoNumbersV1(input1, input2);
+            for (ListNode x1 = answer; x1 != null; x1 = x1?.next)
+                Console.Write(x1.val + " ");
+            Console.Write("\n");
+
+            answer = solution.AddTwoNumbersV2(input1, input2);
             for (ListNode x1 = answer; x1 != null; x1 = x1?.next)
                 Console.Write(x1.val + " ");
             Console.Write("\n");

P002-Add-Two-Numbers/P002-Add-Two-Numbers/Solution.cs

@@ -13,7 +13,7 @@ namespace P002_Add_Two_Numbers
          * Status: Accepted
          * Runtime: 166 ms
          */
-        public ListNode AddTwoNumbers(ListNode l1, ListNode l2)
+        public ListNode AddTwoNumbersV1(ListNode l1, ListNode l2)
         {
             StringBuilder result = new StringBuilder();
 
@@ -40,5 +40,27 @@ namespace P002_Add_Two_Numbers
 
             return head;
         }
+        /**
+         * 1562 / 1562 test cases passed.
+         * Status: Accepted
+         * Runtime: 189 ms
+         */
+        public ListNode AddTwoNumbersV2(ListNode l1, ListNode l2)
+        {
+            ListNode previous = new ListNode(0);
+            ListNode head = previous;
+            int carry = 0, value = 0;
+
+            for (ListNode x1 = l1, x2 = l2; x1 != null || x2 != null || carry != 0; x1 = x1?.next, x2 = x2?.next)
+            {
+                ListNode current = new ListNode(0);
+                value = (x1 != null ? x1.val : 0) + (x2 != null ? x2.val : 0) + carry;
+                carry = value / 10;
+                current.val = (value % 10);
+                previous = previous.next = current;
+            }
+
+            return head.next;
+        }
     }
 }