chore(C): add doubly linked list (#456)

algorithms/C/README.md

@@ -19,6 +19,7 @@
 - [Circular Linked List](linked-lists/Insert-and-del-beginning-circular-ll.c)
 - [Merge two Linked Lists](linked-lists/merge-two-linkedlists.c)
 - [Reverse a Linked List](linked-lists/reverse-linkedlists.c)
+- [Doubly Linked List](linked-lists/doubly-linked-list.c)
 
 ## Queues
 - [Double Ended Queue using array](queues/double-ended-queue-using-array.c)

algorithms/C/linked-lists/doubly-linked-list.c

@@ -0,0 +1,153 @@
+/*
+A doubly linked list is a linked data structure that consists of 
+a set of sequentially linked records called nodes. 
+Each node contains three fields: two link fields 
+(references to the previous and to the next node in the sequence of nodes) 
+and one data field.
+*/
+
+#include <stdio.h>
+#include <stdlib.h>
+
+#define HEAD(x) (x->head)
+#define TAIL(x) (x->tail)
+#define INC_NODE_COUNT(x) (x->count++)
+#define DEC_NODE_COUNT(x) (x->count--)
+#define SIZE(x) (x->count)
+
+typedef struct node_{
+	int val;
+	struct node_* next;
+	struct node_* prev;
+}node;
+
+typedef struct{
+	unsigned int count;
+	node* head;
+	node* tail;
+}dll;
+
+void init(dll* list){
+	list->count = 0;
+	list->head = NULL;
+	list->tail = NULL;
+}
+
+node* createnode(int val){
+	node* n = (node*)malloc(sizeof(node));
+	n->val = val;
+	return n;
+}
+
+void push_front(dll* list, int val){
+	node* new = createnode(val);
+	node* cur = HEAD(list);
+	INC_NODE_COUNT(list);
+	if(cur == NULL){
+		HEAD(list) = new;
+		TAIL(list) = new;
+		return;
+	}
+	cur->prev = new;
+	new->next = cur;
+	list->head = new;
+}
+
+void push_back(dll* list, int val){
+	node* new = createnode(val);
+	node* cur = TAIL(list);
+	INC_NODE_COUNT(list);
+	if(cur == NULL){
+		HEAD(list) = new;
+		TAIL(list) = new;
+		return;
+	}
+	cur->next = new;
+	new->prev = cur;
+	list->tail = new;
+}
+
+int pop_front(dll* list){
+	if(HEAD(list) == NULL){
+		return -1;
+	}
+	node* cur = HEAD(list);
+	int val = cur->val;
+	HEAD(list) = HEAD(list)->next;
+	free(cur);
+	cur = NULL;
+	HEAD(list)->prev = NULL;
+	DEC_NODE_COUNT(list);
+	return val;
+}
+
+int pop_back(dll* list){
+	if(TAIL(list) == NULL){
+		return -1;
+	}
+	node* cur = TAIL(list);
+	int val = cur->val;
+	TAIL(list) = TAIL(list)->prev;
+	free(cur);
+	cur = NULL;
+	TAIL(list)->next = NULL;
+	DEC_NODE_COUNT(list);
+	return val;
+}
+
+void delete(dll* list){
+	if(HEAD(list) == NULL){
+		return;
+	}
+	node* cur = HEAD(list);
+	node* temp = NULL;
+	while(cur != NULL){
+		temp = cur->next;
+		free(cur);
+		cur = NULL;
+		cur = temp;
+	}
+	SIZE(list) = 0;
+}
+
+void print(dll* list){
+	node* cur = list->head;
+	while(NULL != cur){
+		printf("%i ", cur->val);
+		cur = cur->next;
+	}
+}
+
+int main(){
+	dll* list = (dll*)malloc(sizeof(dll));
+	init(list);
+
+	push_front(list, 3);
+	push_front(list, 2);
+	push_front(list, 1);
+
+	push_back(list, 4);
+	push_back(list, 5);
+	push_back(list, 6);
+
+	pop_front(list);
+	pop_back(list);
+
+	print(list);
+
+	delete(list);
+	
+	/*
+	
+	Time Complexity
+	---------------
+	push_front: O(1)
+	push_back: O(1)
+	pop_front: O(1)
+	pop_back: O(1)
+	delete: O(N)
+	
+	*/
+	
+	return 0;
+}