refactory the code

algorithms/cpp/sortList/sortList.cpp

@@ -28,9 +28,7 @@ ListNode *sortList(ListNode *head) {
     //find the middle place
     ListNode *p1=head, *p2=head->next; 
 
-    ListNode *prev;
     while(p2 && p2->next){
-        prev = p1;
         p1 = p1->next;
         p2 = p2->next->next;
     }
@@ -44,23 +42,20 @@ ListNode *mergeTwoLists(ListNode* head1, ListNode* head2){
     ListNode *p1 = head1,  *p2=head2;
     static ListNode dummy(0);
 
-    dummy.next = p1;
-    ListNode *prev = &dummy;
+    ListNode *tail = &dummy;
 
     while(p1 && p2){
         if(p1->val < p2->val){
-            prev = p1;
+            tail->next = p1;
             p1 = p1->next;
         }else{
-            prev->next = p2;
+            tail->next = p2;
             p2 = p2->next;
-            prev = prev->next;
-            prev->next = p1;
         }
+        tail = tail->next;
     }
-    if (p2){
-        prev->next = p2;
-    }
+    if (p1) tail->next = p1;
+    if (p2) tail->next = p2;
 
     return dummy.next;
 }
@@ -101,11 +96,11 @@ int main(int argc, char** argv)
     for(int i=0; i<n; i++){
         a[i] = random()%n + 1;
     }
-    
+
     ListNode *p = createList(a, n);
     printList(p);
     printList(sortList(p));
-    
+
     delete[] a;
     return 0;
 }