docs(en): add doubly linked list (MakeContributions#813)

Co-authored-by: Ming Tsai <[email protected]>

docs/en/Lists/doubly-linked-list.md

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+# Doubly Linked List
+
+A doubly linked list is a type of linked data structure that allows traversal in both directions (forward and backward directions). 
+It differs from singly linked list in that, singly linked list does not support backward traversal. 
+Every node in a doubly linked list contains a **data** part and two pointers; **next pointer** that points to the next node and the **prev pointer** that points to the previous node in the list. 
+A doubly linked list also has two dummy nodes (head and tail) known as **sentinels** (guard nodes) that contains no data of the sequence. 
+The first element of a doubly linked list has its prev pointer pointing to head and the last element has its next node pointing to tail. 
+
+Without a tail pointer, you cannot directly access the last element without having to first traverse the entire list all the way from the beginning. 
+
+## Steps to create a doubly linked list
+To create an empty list,
+- create head and tail nodes (nodes without data) first
+- let the **next pointer** of the head point to the tail node
+- let the **prev pointer** of the tail point to the head node.
+
+For a non-empty list,
+- let the next pointer of the head point to the first real element of the sequence
+- let the prev pointer of the tail point to the last real element of the sequence
+
+## Key operations
+- Insertion (insert at the beginning, at the end or anywhere in the middle)
+- deletion (delete from beginning, end or anywhere in the middle)
+
+To add an element(node) at the beginning of the list, let the;
+- *next pointer* of the new element (node) point to the first element (the one the *head* currently points to)
+- *next pointer* of the *head* point to this new element you want to add
+- *prev pointer* of the element (node) you want to add point to *head*.
+
+To add an alement (node) to the end of the list, let the;
+- *prev pointer* of the new element you want to add point to the last element (the one before the *tail*)
+- *next pointer* of the last element (the one before the *tail*) point to the new element you want to add
+- *prev pointer* of the *tail* point to the new element.
+
+To delete an element from the beginning, let the;
+- *next pointer* of the *head* point to the second element in the list (node after the one *head* pointers to). 
+- *prev pointer* of the second element point to the *head*
+
+
+To delete from the end of the list, let the;
+- *prev pointer* of the *tail* point to the element before the last node
+- *next pointer* of the element before the last node point to the *tail*
+
+Note: If there is only one element in the list, deletion from both directions happens by pointing the *next pointer* of *head* to *tail* and *prev pointer* of *tail* to *head*. 
+
+Deletion anywhere in the middle happens by letting the *prev pointer* and the *next pointer* of the node after and before the one you want to delete point to each other respectively.
+
+To insert anywhere in the middle, let the;
+- *next pointer* of the element you want to insert point to the node at the position you want to insert it
+- *prev pointer* of the element you want to insert point to node before the position you want to insert it
+- *prev pointer* of the node at the position you want to insert point to the new element
+- *next pointer* of the node before the position you want to insert point to the new element.
+
+Deletion and insertion at the beginning or end of the list is a constant operation and has a O(1) runtime. Deletion or insertion anywhere in the middle however has a O(n) runtime. 
+
+
+## Implementation
+
+- [C++](../../../algorithms/C/linked-lists/doubly-linked-list.c)
+- [Java](../../../algorithms/CPlusPlus/Linked-Lists/doubly.cpp)
+- [JavaScript](../../../algorithms/Java/linked-lists/doubly.java)
+- [Python](../../../algorithms/Python/linked_lists/doubly.py)
+
+## Video URL
+
+[Doubly linked list](https://www.youtube.com/watch?v=nquQ_fYGGA4)
+
+
+## References
+
+[Goodrich, T. M., Tamassia, R., Goldwasser, M.H.(2014). *Data Structures & Algorithms in Java* (6th ed.). Wiley](https://www.wiley.com/en-us/Data+Structures+and+Algorithms+in+Java,+6th+Edition-p-9781118771334)

docs/en/README.md

@@ -1,5 +1,9 @@
 # Algorithms
 
+## Lists
+- [Singly linked list](./Lists/singly-linked-list.md)
+- [Doubly linked list](./Lists/doubly-linked-list.md)
+
 ## Sorting
 
 - [Bubble Sort](./Sorting/Bubble-Sort.md)