021 fix: if one of the given lists is null

Li Beibei · Li Beibei · commit f1c5a624b7af · 2019-04-25T16:47:29.000+08:00
diff --git a/note/021/README.md b/note/021/README.md
@@ -31,6 +31,11 @@ class Solution {
     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
         ListNode head = new ListNode(0);
         ListNode temp = head;
+	if (l1 == null) {
+            return l2;
+        } else if (l2 == null) {
+            return l1;
+        }
         while (l1 != null && l2 != null) {
             if (l1.val < l2.val) {
                 temp.next = l1;
