Add Doubly Linked List DS (NITSkmOS#468)

doubly_linked_list.c: Add Doubly Linked List

This adds Doubly Linked List that can be traversed in both forward and
backward directions. There are functions that insert nodes and prints
the list.

Closes NITSkmOS#31

README.md

@@ -38,7 +38,7 @@ This repository contains examples of various algorithms written on different pro
 | [AVL Tree](https://en.wikipedia.org/wiki/AVL_tree)                                              | [:octocat:](avl_tree/C)               | [:octocat:](avl_tree/Cpp)      | [:octocat:](avl_tree/Java)            | [:octocat:](avl_tree/Python)          |
 | [Binary Search Tree](https://en.wikipedia.org/wiki/Binary_search_tree)                          |                                       | [:octocat:](binary_search_tree/Cpp)                                       |                                       |                                       |
 | [Fenwick Tree](https://en.wikipedia.org/wiki/Fenwick_tree)                                      |                                       |                                       | [:octocat:](fenwick_tree/java)        | [:octocat:](fenwick_tree/Python)      |
-
+| [Doubly Linked List](https://en.wikipedia.org/wiki/Doubly_linked_list)                          | [:octocat:](doubly_linked_list/c)     |                                       |                                       |                                       |
 
 ## Sample Run
 

doubly_linked_list/c/doubly_linked_list.c

@@ -0,0 +1,73 @@
+#include <stdio.h>
+#include <stdlib.h>
+
+//Node struct for the Linked List
+struct Node {
+	int data;
+	struct Node* next;
+	struct Node* prev;
+};
+
+//An empty list is created.
+void create_list(struct Node **head_ref) {
+	*head_ref = NULL;
+	printf("List created successfully.");
+	return;
+}
+
+/* Given reference to the head of the list, this function
+inserts a node at the beginning of the List*/
+void insert(struct Node **head_ref, int new_data) {
+	//Allocate memory for the node
+	struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
+	new_node->data = new_data;
+	new_node->next = (*head_ref);
+	new_node->prev = NULL;
+	if ((*head_ref) != NULL)
+		(*head_ref)->prev = new_node;
+	(*head_ref) = new_node;
+}
+
+/*Given reference to the head of the list, insert_before()
+adds a node before a given node*/
+void insert_before(struct Node *next_node, int new_data) {
+	if (next_node == NULL) {
+		printf("The given Next node is NULL");
+		return;
+	}
+	struct Node* new_node = (struct Node*)malloc(sizeof(struct Node));
+	new_node->data = new_data;
+	new_node->prev = next_node->prev;
+	next_node->prev = new_node;
+	new_node->next = next_node;
+	if (new_node->prev != NULL)
+		new_node->prev->next = new_node;
+}
+
+// Print the list in both directions
+void print_list(struct Node *node) {
+	struct Node* last;
+	printf("\nTraversal in forward direction \n");
+	while (node != NULL) {
+		printf(" %d ", node->data);
+		last = node;
+		node = node->next;
+	}
+	printf("\nTraversal in reverse direction \n");
+	while (last != NULL) {
+		printf(" %d ", last->data);
+		last = last->prev;
+	}
+	printf("\n");
+}
+
+int main() {
+	struct Node *head;
+	create_list(&head);
+	insert(&head, 7);
+	insert(&head, 1);
+	insert(&head, 4);
+	insert_before(head->next, 8);
+	print_list(head);
+	return 0;
+}