updating DIRECTORY.md

DIRECTORY.md

@@ -33,6 +33,7 @@
   * [Queue Using Array2](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_array2.cpp)
   * [Queue Using Linked List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_linked_list.cpp)
   * [Queue Using Linkedlist](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_linkedlist.cpp)
+  * [Queue Using Two Stacks](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/queue_using_two_stacks.cpp)
   * [Skip List](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/skip_list.cpp)
   * [Stack](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack.h)
   * [Stack Using Array](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/stack_using_array.cpp)