impl insert and merge

List.h

@@ -78,8 +78,12 @@ int length(List *list) {
     return _length(list->head);
 }
 
-void insert(List *list, int position) {
+void insert(List *list, void* elem, int position) {
+    Node* insertpos = _traverseToPos(list->head, position);
+    Node* newnode = _newNode(insertpos->next, insertpos, elem);
 
+    insertpos->next = newnode;
+    insertpos->next->next->prev = newnode;
 }
 
 void delete(List *list, int position) {
@@ -113,16 +117,19 @@ void deleteList(List *list) {
     free(list);
 }
 
+Node *first(List *list) {
+    return list->head;
+}
+
+Node *last(List *list) {
+    return _traverseToLast(list->head);
+}
+
 List *merge(List *list1, List *list2) {
-    return NULL;
-}
-
-void *first(List *list) {
-    return list->head->dta;
-}
-
-void *last(List *list) {
-    return _traverseToLast(list->head)->dta;
+    Node* end = last(list1);
+    end->next = list2->head;
+    list2->head->prev = end;
+    return list1;
 }
 
 void print(List *list, char *typeFormatter) {