Add imports for Java solutions

solutions/add-two-numbers/Solution.java

@@ -3,16 +3,17 @@
 // Memory Usage: 39.3 MB
 // Submitted: January 15th, 2021
 
-/**
- * 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 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 addTwoNumbers(ListNode a, ListNode b) {
         ListNode dummy = new ListNode(0);