# Linked List - Reverse the List

While working with a Linked List, sometimes it is required to reverse it. Reversing a List produces following result: if the given List is 10->20->30->40->50, after reversing the List the List becomes 50->40->30->20->10.

Reversing a List requires creating three nodes, considering that the list is not empty, which are as follows: tempNode pointing to head, prevNode pointing to head and curNode pointing to next of head. Then make next of prevNode as null to make the first node as last node of the reversed list. After that, while the curNode is not null adjust links (unlink curNode and link it to the reversed list from front and modify curNode and prevNode to the next element in the list). At last, make the prevNode (last node) as head.

The function reverseList is created for this purpose. It is a 4-step process.

```void reverseList() {
//1. If head is not null create three nodes
//   prevNode - pointing to head,
//   tempNode - pointing to head,
//   curNode - pointing to next of head
if(head != NULL) {
Node* prevNode = head;
Node* tempNode = head;
Node* curNode = head->next;

//2. assign next of prevNode as null to make the
//   first node as last node of the reversed list
prevNode->next = NULL;

while(curNode != NULL) {
//3. While the curNode is not null adjust links
//   (unlink curNode and link it to the reversed list
//   from front and modify curNode and prevNode)
tempNode = curNode->next;
curNode->next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

//4. Make prevNode (last node) as head
head = prevNode;
}
}
```
```void reverseList(struct Node** head_ref) {
//1. If head is not null create three nodes
//   prevNode - pointing to head,
//   tempNode - pointing to head,
//   curNode - pointing to next of head
if(*head_ref != NULL) {
struct Node* prevNode = *head_ref;
struct Node* tempNode = *head_ref;
struct Node* curNode = (*head_ref)->next;

//2. assign next of prevNode as null to make the
//   first node as last node of the reversed list
prevNode->next = NULL;

while(curNode != NULL) {
//3. While the curNode is not null adjust links
//   (unlink curNode and link it to the reversed list
//   from front and modify curNode and prevNode)
tempNode = curNode->next;
curNode->next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

//4. Make prevNode (last node) as head
*head_ref = prevNode;
}
}
```
```def reverseList(self):
#1. If head is not null create three nodes
#   prevNode - pointing to head,
#   tempNode - pointing to head,
#   curNode - pointing to next of head
if(self.head != None):
prevNode = self.head
tempNode = self.head
curNode = self.head.next

#2. assign next of prevNode as null to make the
#   first node as last node of the reversed list
prevNode.next = None

while(curNode != None):
#3. While the curNode is not null adjust links
#   (unlink curNode and link it to the reversed list
#   from front and modify curNode and prevNode)
tempNode = curNode.next
curNode.next = prevNode
prevNode = curNode
curNode = tempNode

#4. Make prevNode (last node) as head
self.head = prevNode
```
```void reverseList() {
//1. If head is not null create three nodes
//   prevNode - pointing to head,
//   tempNode - pointing to head,
//   curNode - pointing to next of head
if(this.head != null) {
Node prevNode = this.head;
Node tempNode = this.head;
Node curNode = this.head.next;

//2. assign next of prevNode as null to make the
//   first node as last node of the reversed list
prevNode.next = null;

while(curNode != null) {
//3. While the curNode is not null adjust links
//   (unlink curNode and link it to the reversed list
//   from front and modify curNode and prevNode)
tempNode = curNode.next;
curNode.next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

//4. Make prevNode (last node) as head
this.head = prevNode;
}
}
```
```public void reverseList() {
//1. If head is not null create three nodes
//   prevNode - pointing to head,
//   tempNode - pointing to head,
//   curNode - pointing to next of head
if(this.head != null) {
Node prevNode = this.head;
Node tempNode = this.head;
Node curNode = this.head.next;

//2. assign next of prevNode as null to make the
//   first node as last node of the reversed list
prevNode.next = null;

while(curNode != null) {
//3. While the curNode is not null adjust links
//   (unlink curNode and link it to the reversed list
//   from front and modify curNode and prevNode)
tempNode = curNode.next;
curNode.next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

//4. Make prevNode (last node) as head
this.head = prevNode;
}
}
```
```public function reverseList() {
//1. If head is not null create three nodes
//   prevNode - pointing to head,
//   tempNode - pointing to head,
//   curNode - pointing to next of head
if(\$this->head != null) {
\$prevNode = \$this->head;
\$tempNode = \$this->head;
\$curNode = \$this->head->next;

//2. assign next of prevNode as null to make the
//   first node as last node of the reversed list
\$prevNode->next = null;

while(\$curNode != null) {
//3. While the curNode is not null adjust links
//   (unlink curNode and link it to the reversed list
//   from front and modify curNode and prevNode)
\$tempNode = \$curNode->next;
\$curNode->next = \$prevNode;
\$prevNode = \$curNode;
\$curNode = \$tempNode;
}

//4. Make prevNode (last node) as head
\$this->head = \$prevNode;
}
}
```

The below is a complete program that uses above discussed concept to reverse a given linked list.

```#include <iostream>
using namespace std;

//node structure
struct Node {
int data;
Node* next;
};

class LinkedList {
private:
Node* head;
public:
LinkedList(){
head = NULL;
}

//Add new element at the end of the list
void push_back(int newElement) {
Node* newNode = new Node();
newNode->data = newElement;
newNode->next = NULL;
if(head == NULL) {
head = newNode;
} else {
Node* temp = head;
while(temp->next != NULL)
temp = temp->next;
temp->next = newNode;
}
}

//reverse the list
void reverseList() {
if(head != NULL) {
Node* prevNode = head;
Node* tempNode = head;
Node* curNode = head->next;

prevNode->next = NULL;

while(curNode != NULL) {
tempNode = curNode->next;
curNode->next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

head = prevNode;
}
}

//display the content of the list
void PrintList() {
Node* temp = head;
if(temp != NULL) {
cout<<"The list contains: ";
while(temp != NULL) {
cout<<temp->data<<" ";
temp = temp->next;
}
cout<<"\n";
} else {
cout<<"The list is empty.\n";
}
}
};

// test the code
int main() {
LinkedList MyList;

//Add five elements in the list.
MyList.push_back(10);
MyList.push_back(20);
MyList.push_back(30);
MyList.push_back(40);
MyList.push_back(50);

//Display the content of the list.
MyList.PrintList();

//Reversing the list.
MyList.reverseList();

//Display the content of the list.
MyList.PrintList();

return 0;
}
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```
```#include <stdio.h>
#include <stdlib.h>

//node structure
struct Node {
int data;
struct Node* next;
};

//Add new element at the end of the list
void push_back(struct Node** head_ref, int newElement) {
struct Node *newNode, *temp;
newNode = (struct Node*)malloc(sizeof(struct Node));
newNode->data = newElement;
newNode->next = NULL;
if(*head_ref == NULL) {
*head_ref = newNode;
} else {
temp = *head_ref;
while(temp->next != NULL) {
temp = temp->next;
}
temp->next = newNode;
}
}

//reverse the list
void reverseList(struct Node** head_ref) {
if(*head_ref != NULL) {
struct Node* prevNode = *head_ref;
struct Node* tempNode = *head_ref;
struct Node* curNode = (*head_ref)->next;

prevNode->next = NULL;

while(curNode != NULL) {
tempNode = curNode->next;
curNode->next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

*head_ref = prevNode;
}
}

//display the content of the list
void PrintList(struct Node* head_ref) {
struct Node* temp = head_ref;
if(head_ref != NULL) {
printf("The list contains: ");
while (temp != NULL) {
printf("%i ",temp->data);
temp = temp->next;
}
printf("\n");
} else {
printf("The list is empty.\n");
}
}

// test the code
int main() {
struct Node* MyList = NULL;

//Add five elements in the list.
push_back(&MyList, 10);
push_back(&MyList, 20);
push_back(&MyList, 30);
push_back(&MyList, 40);
push_back(&MyList, 50);

//Display the content of the list.
PrintList(MyList);

//Reversing the list.
reverseList(&MyList);

//Display the content of the list.
PrintList(MyList);

return 0;
}
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```
```# node structure
class Node:
def __init__(self, data):
self.data = data
self.next = None

#class Linked List
class LinkedList:
def __init__(self):
self.head = None

#Add new element at the end of the list
def push_back(self, newElement):
newNode = Node(newElement)
if(self.head == None):
self.head = newNode
return
else:
temp = self.head
while(temp.next != None):
temp = temp.next
temp.next = newNode

#reverse the list
def reverseList(self):
if(self.head != None):
prevNode = self.head
tempNode = self.head
curNode = self.head.next

prevNode.next = None

while(curNode != None):
tempNode = curNode.next
curNode.next = prevNode
prevNode = curNode
curNode = tempNode

self.head = prevNode

#display the content of the list
def PrintList(self):
temp = self.head
if(temp != None):
print("The list contains:", end=" ")
while (temp != None):
print(temp.data, end=" ")
temp = temp.next
print()
else:
print("The list is empty.")

# test the code
MyList = LinkedList()

#Add five elements in the list.
MyList.push_back(10)
MyList.push_back(20)
MyList.push_back(30)
MyList.push_back(40)
MyList.push_back(50)

#Display the content of the list.
MyList.PrintList()

#Reversing the list.
MyList.reverseList()

#Display the content of the list.
MyList.PrintList()
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```
```//node structure
class Node {
int data;
Node next;
};

class LinkedList {
Node head;

LinkedList(){
head = null;
}

//Add new element at the end of the list
void push_back(int newElement) {
Node newNode = new Node();
newNode.data = newElement;
newNode.next = null;
if(head == null) {
head = newNode;
} else {
Node temp = new Node();
temp = head;
while(temp.next != null)
temp = temp.next;
temp.next = newNode;
}
}

//reverse the list
void reverseList() {
if(this.head != null) {
Node prevNode = this.head;
Node tempNode = this.head;
Node curNode = this.head.next;

prevNode.next = null;

while(curNode != null) {
tempNode = curNode.next;
curNode.next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

this.head = prevNode;
}
}

//display the content of the list
void PrintList() {
Node temp = new Node();
temp = this.head;
if(temp != null) {
System.out.print("The list contains: ");
while(temp != null) {
System.out.print(temp.data + " ");
temp = temp.next;
}
System.out.println();
} else {
System.out.println("The list is empty.");
}
}
};

// test the code
public class Implementation {
public static void main(String[] args) {
LinkedList MyList = new LinkedList();

//Add five elements in the list.
MyList.push_back(10);
MyList.push_back(20);
MyList.push_back(30);
MyList.push_back(40);
MyList.push_back(50);

//Display the content of the list.
MyList.PrintList();

//Reversing the list.
MyList.reverseList();

//Display the content of the list.
MyList.PrintList();
}
}
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```
```using System;

//node structure
class Node {
public int data;
public Node next;
};

class LinkedList {
Node head;

public LinkedList(){
head = null;
}

//Add new element at the end of the list
public void push_back(int newElement) {
Node newNode = new Node();
newNode.data = newElement;
newNode.next = null;
if(head == null) {
head = newNode;
} else {
Node temp = new Node();
temp = head;
while(temp.next != null)
temp = temp.next;
temp.next = newNode;
}
}

//reverse the list
public void reverseList() {
if(this.head != null) {
Node prevNode = this.head;
Node tempNode = this.head;
Node curNode = this.head.next;

prevNode.next = null;

while(curNode != null) {
tempNode = curNode.next;
curNode.next = prevNode;
prevNode = curNode;
curNode = tempNode;
}

this.head = prevNode;
}
}

//display the content of the list
public void PrintList() {
Node temp = new Node();
temp = this.head;
if(temp != null) {
Console.Write("The list contains: ");
while(temp != null) {
Console.Write(temp.data + " ");
temp = temp.next;
}
Console.WriteLine();
} else {
Console.WriteLine("The list is empty.");
}
}
};

// test the code
class Implementation {
static void Main(string[] args) {
LinkedList MyList = new LinkedList();

//Add five elements in the list.
MyList.push_back(10);
MyList.push_back(20);
MyList.push_back(30);
MyList.push_back(40);
MyList.push_back(50);

//Display the content of the list.
MyList.PrintList();

//Reversing the list.
MyList.reverseList();

//Display the content of the list.
MyList.PrintList();
}
}
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```
```<?php
//node structure
class Node {
public \$data;
public \$next;
}

class LinkedList {
public \$head;

public function __construct(){
\$this->head = null;
}

//Add new element at the end of the list
public function push_back(\$newElement) {
\$newNode = new Node();
\$newNode->data = \$newElement;
\$newNode->next = null;
if(\$this->head == null) {
\$this->head = \$newNode;
} else {
\$temp = new Node();
\$temp = \$this->head;
while(\$temp->next != null) {
\$temp = \$temp->next;
}
\$temp->next = \$newNode;
}
}

//reverse the list
public function reverseList() {
if(\$this->head != null) {
\$prevNode = \$this->head;
\$tempNode = \$this->head;
\$curNode = \$this->head->next;

\$prevNode->next = null;

while(\$curNode != null) {
\$tempNode = \$curNode->next;
\$curNode->next = \$prevNode;
\$prevNode = \$curNode;
\$curNode = \$tempNode;
}

\$this->head = \$prevNode;
}
}

//display the content of the list
public function PrintList() {
\$temp = new Node();
\$temp = \$this->head;
if(\$temp != null) {
echo "The list contains: ";
while(\$temp != null) {
echo \$temp->data." ";
\$temp = \$temp->next;
}
echo "\n";
} else {
echo "The list is empty.\n";
}
}
};

// test the code
\$MyList = new LinkedList();

//Add five elements in the list.
\$MyList->push_back(10);
\$MyList->push_back(20);
\$MyList->push_back(30);
\$MyList->push_back(40);
\$MyList->push_back(50);

//Display the content of the list.
\$MyList->PrintList();

//Reversing the list.
\$MyList->reverseList();

//Display the content of the list.
\$MyList->PrintList();
?>
```

The above code will give the following output:

```The list contains: 10 20 30 40 50
The list contains: 50 40 30 20 10
```

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