adding notes on DLList.

Author: Clément

source/lectures/data/DLList.md

@@ -0,0 +1,20 @@
+---
+tags:
+- datatypes/collections
+---
+
+# Doubly Linked List
+
+Here is *yet* another implementation of the list abstract data type, using *doubly linked list*.
+
+```{download="./code/projects/DLList_ICollection.zip"}
+!include code/projects/DLList_ICollection/DLList_ICollection/DLList.cs
+```
+
+The main differences with *singly* linked list are as follows:
+
+- Instead of keeping only track of the first element (`first`), we keep track of both the first (`head`) and last (`tail`) elements.
+- Each `Cell` contains a pointer to the "next" element (as before), but also to the "previous" element.
+- Adding (or removing) to the right is now done in constant time, instead of linear time.
+- Traversing the list in opposite order (from end to beginning) is now straightforward (cf. the `ToString` method above).
+- The rest of the edits are about bookkeeping the `Previous` and `Next` attributes of the `Cell`, as well as updating the `tail` attribute.

source/order

@@ -61,6 +61,7 @@
 ./lectures/data/lists.md
 ./lectures/data/CList.md
 ./lectures/data/CList_as_ICollection.md
+./lectures/data/DLList.md
 ./lectures/misc/
 ./lectures/misc/over_under_flow.md
 ./lectures/misc/random.md