Optimize List to array conversion

src/include/List.h

 /*! \file List.h
  */
 
+#ifndef NULL
+#define NULL 0
+#endif
 #ifndef LIST_OUT_OF_BOUNDS_EXCEPTION
 #define LIST_OUT_OF_BOUNDS_EXCEPTION 1
 #endif
@@ -154,8 +157,8 @@ template<class T> class List {
   T* to_array() {
     T *array = new T[size];
     current = last;
-    for (int i = size; i > 0; i--) {
-      array[i - 1] = current->value;
+    for (int i = size - 1; i > -1; i--) {
+      array[i] = current->value;
       current = current->p_prev;
     }
     return array;