add

cv5/a_train.c

@@ -43,15 +43,13 @@ void cancel_train(struct car* first) {
     if(first == NULL){
         exit(0);
     }
-    if(first->next == NULL){
-       struct car* this = first;
-       first = first->next;
-       free(this);
-       first = NULL;
+    else if(first->next == NULL){
+       free(first);
+       //first = NULL;
     }
     else{    
         cancel_train(first->next);
-        free(first);
+        first->next = NULL;
     }
 
 }