From f9f30871c2c93cad993ebea33ca1e73c6a52cecc Mon Sep 17 00:00:00 2001
From: Mahi Yadav <95998757+Mahi5902@users.noreply.github.com>
Date: Wed, 4 Oct 2023 20:24:07 +0530
Subject: [PATCH] Reverse_a_doubly_linked_list.cpp

here is my code in c++ for reversing a doubly linked list with proper comments and also i have explained my approach with time complexity and space complexity.
---
 Reverse_a_doubly_linked_list.cpp | 114 +++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 Reverse_a_doubly_linked_list.cpp

diff --git a/Reverse_a_doubly_linked_list.cpp b/Reverse_a_doubly_linked_list.cpp
new file mode 100644
index 0000000..bf19c59
--- /dev/null
+++ b/Reverse_a_doubly_linked_list.cpp
@@ -0,0 +1,114 @@
+/* C++ program to reverse a doubly linked list */
+// approach
+// Follow the given steps to solve the problem :
+
+    // Traverse the linked list using a pointer
+    // Swap the prev and next pointers for all nodes
+    // At last, change the head pointer of the doubly linked list
+
+#include <bits/stdc++.h>
+using namespace std;
+
+/* Node of the doubly linked list */
+class Node {
+public:
+	int data;
+	Node* next;
+	Node* prev;
+};
+
+/* Function to reverse a Doubly Linked List */
+
+void reverse(Node** head_ref)
+{
+	Node* temp = NULL;
+	Node* current = *head_ref;
+
+	/* swap next and prev for all nodes of
+	doubly linked list */
+  
+	while (current != NULL) {
+		temp = current->prev;
+		current->prev = current->next;
+		current->next = temp;
+		current = current->prev;
+	}
+
+	/* Before changing the head, check for the cases like
+	empty list and list with only one node */
+  
+	if (temp != NULL)
+		*head_ref = temp->prev;
+}
+
+/* UTILITY FUNCTIONS */
+/* Function to insert a node at the
+beginning of the Doubly Linked List */
+
+void push(Node** head_ref, int new_data)
+{
+	/* allocate node */
+  
+	Node* new_node = new Node();
+
+	/* put in the data */
+	new_node->data = new_data;
+
+	/* since we are adding at the beginning,
+	prev is always NULL */
+  
+	new_node->prev = NULL;
+
+	/* link the old list of the new node */
+	new_node->next = (*head_ref);
+
+	/* change prev of head node to new node */
+	if ((*head_ref) != NULL)
+		(*head_ref)->prev = new_node;
+
+	/* move the head to point to the new node */
+	(*head_ref) = new_node;
+}
+
+/* Function to print nodes in a given doubly linked list
+This function is same as printList() of singly linked list
+*/
+
+void printList(Node* node)
+{
+	while (node != NULL) {
+		cout << node->data << " ";
+		node = node->next;
+	}
+}
+
+// main code
+int main()
+{
+	/* Start with the empty list */
+	Node* head = NULL;
+
+	/* Let us create a sorted linked list to test the
+	functions Created linked list will be 10->8->4->2 */
+	push(&head, 2);
+	push(&head, 4);
+	push(&head, 8);
+	push(&head, 10);
+
+	cout << "Original Linked list" << endl;
+	printList(head);
+
+	// Function call
+	reverse(&head);
+
+	cout << "\nReversed Linked list" << endl;
+	printList(head);
+
+	return 0;
+}
+
+// output
+ // Original Linked list 10 8 4 2 
+ // Reversed Linked list 2 4 8 10 
+// Time Complexity: O(N), where N denotes the number of nodes in the doubly linked list.
+// Auxiliary Space: O(1)