Code for inserting elements in doubly linked list (#67)

* Code for inserting elements in doubly linked list

* Circular linked list added

linked-lists/README.md

@@ -5,6 +5,10 @@
 1. [Singly Linked List](c-or-cpp/singly.cpp)
 2. [Reversing Linked List](c-or-cpp/reverse-linkedlist.cpp)
 
+2. [Doubly Linked List](c-or-cpp/doubly.cpp)
+
+3. [Circular Linked List](c-or-cpp/circular.cpp)
+
 ### Java
 
 1. [Singly Linked List](java/singly.cpp)

linked-lists/c-or-cpp/circular.cpp

@@ -0,0 +1,39 @@
+#include <iostream>
+using namespace std;
+struct Node {
+   int data;
+   struct Node *next;
+};
+struct Node* head = NULL;
+void insert(int newdata) {
+   struct Node *newnode = (struct Node *)malloc(sizeof(struct Node));
+   struct Node *ptr = head;
+   newnode->data = newdata;
+   newnode->next = head;
+   if (head!= NULL) {
+      while (ptr->next != head)
+      ptr = ptr->next;
+      ptr->next = newnode;
+   } else
+   newnode->next = newnode;
+   head = newnode;
+}
+void display() {
+   struct Node* ptr;
+   ptr = head;
+   do {
+      cout<<ptr->data <<" ";
+      ptr = ptr->next;
+   } while(ptr != head);
+}
+int main() {
+   insert(3);
+   insert(1);
+   insert(7);
+   insert(3);
+   insert(1);
+   insert(7);
+   cout<<"The circular linked list is: ";
+   display();
+   return 0;
+}

linked-lists/c-or-cpp/doubly.cpp

@@ -0,0 +1,36 @@
+#include <iostream>
+#include<bits/stdc++.h>
+using namespace std;
+struct Node {
+   int data;
+   struct Node *prev;
+   struct Node *next;
+};
+struct Node* head = NULL;
+void insert(int newdata) {
+   struct Node* newnode = (struct Node*) malloc(sizeof(struct Node));
+   newnode->data = newdata;
+   newnode->prev = NULL;
+   newnode->next = head;
+   if(head != NULL)
+   head->prev = newnode ;
+   head = newnode;
+}
+void display() {
+   struct Node* ptr;
+   ptr = head;
+   while(ptr != NULL) {
+      cout<< ptr->data <<" ";
+      ptr = ptr->next;
+   }
+}
+int main() {
+   insert(3);
+   insert(1);
+   insert(7);
+   insert(2);
+   insert(9);
+   cout<<"The doubly linked list is: ";
+   display();
+   return 0;
+}