Update solution 021 with cpp version

Author: xiapengchng

solution/021.Merge Two Sorted Lists/Solution.cpp

@@ -0,0 +1,37 @@
+/**
+ * Definition for singly-linked list.
+ * struct ListNode {
+ *     int val;
+ *     ListNode *next;
+ *     ListNode(int x) : val(x), next(NULL) {}
+ * };
+ */
+class Solution {
+public:
+    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
+        ListNode *a1=new ListNode(0);
+        ListNode *head=a1;
+        while(l1!=NULL&&l2!=NULL)
+        {
+            if(l1->val<l2->val)
+            {
+                a1->next=l1;
+                l1=l1->next;
+                a1=a1->next;
+            }
+            else
+            {
+                a1->next=l2;
+                l2=l2->next;
+                a1=a1->next;
+            }
+        }
+        if(l1==NULL)
+            a1->next=l2;
+        if(l2==NULL)
+            a1->next=l1;
+        return head->next;
+        
+        
+    }
+};