Reverse a linked list

Given pointer to the head node of a linked list, the task is to reverse the linked list. We need to reverse the list by changing links between nodes.

Examples:

Input : Head of following linked list  
       1->2->3->4->NULL
Output : Linked list should be changed to,
       4->3->2->1->NULL

Input : Head of following linked list  
       1->2->3->4->5->NULL
Output : Linked list should be changed to,
       5->4->3->2->1->NULL

Input : NULL
Output : NULL

Input  : 1->NULL
Output : 1->NULL


Iterative Method

  1. Initialize three pointers prev as NULL, curr as head and next as NULL.
  2. Iterate trough the linked list. In loop, do following.
    // Before changing next of current,
    // store next node
    next = curr->next

    // Now change next of current
    // This is where actual reversing happens
    curr->next = prev

    // Move prev and curr one step forward
    prev = curr
    curr = next

C++

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// Iterative C++ program to reverse
// a linked list
#include<iostream>
using namespace std;
  
/* Link list node */
struct Node
{
    int data;
    struct Node* next;
    Node (int data)
    {
        this->data = data;
        next = NULL;
    }
};
  
struct LinkedList
{
    Node *head;
    LinkedList()
    {
        head = NULL;
    }
  
  
    /* Function to reverse the linked list */
    void reverse()
    {
        // Initialize current, previous and
        // next pointers
        Node *current = head;
        Node *prev = NULL, *next = NULL;
  
  
        while (current != NULL)
        {
            // Store next
            next = current->next;
  
            // Reverse current node's pointer
            current->next = prev;
  
            // Move pointers one position ahead.
            prev = current;
            current = next;
        }
        head = prev;
    }
  
    /* Function to print linked list */
    void print()
    {
        struct Node *temp = head;
        while (temp != NULL)
        {
            cout << temp->data << " ";
            temp = temp->next;
        }
    }
  
    void push(int data)
    {
        Node *temp = new Node(data);
        temp->next = head;
        head = temp;
    }
};
  
/* Driver program to test above function*/
int main()
{
    /* Start with the empty list */
    LinkedList ll;
    ll.push(20);
    ll.push(4);
    ll.push(15);
    ll.push(85);
  
    cout << "Given linked list\n";
    ll.print();
  
    ll.reverse();
  
    cout << "\nReversed Linked list \n";
    ll.print();
    return 0;
}

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C

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// Iterative C program to reverse a linked list
#include<stdio.h>
#include<stdlib.h>
  
/* Link list node */
struct Node
{
    int data;
    struct Node* next;
};
  
/* Function to reverse the linked list */
static void reverse(struct Node** head_ref)
{
    struct Node* prev   = NULL;
    struct Node* current = *head_ref;
    struct Node* next = NULL;
    while (current != NULL)
    {
        // Store next
        next  = current->next;  
  
        // Reverse current node's pointer
        current->next = prev;   
  
        // Move pointers one position ahead.
        prev = current;
        current = next;
    }
    *head_ref = prev;
}
  
/* Function to push a node */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node =
            (struct Node*) malloc(sizeof(struct Node));           
    new_node->data  = new_data;
    new_node->next = (*head_ref);    
    (*head_ref)    = new_node;
}
  
/* Function to print linked list */
void printList(struct Node *head)
{
    struct Node *temp = head;
    while(temp != NULL)
    {
        printf("%d  ", temp->data);    
        temp = temp->next;  
    }
}    
  
/* Driver program to test above function*/
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;
    
     push(&head, 20);
     push(&head, 4);
     push(&head, 15); 
     push(&head, 85);      
      
     printf("Given linked list\n");
     printList(head);    
     reverse(&head);                      
     printf("\nReversed Linked list \n");
     printList(head);    
     getchar();
}

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Java

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// Java program for reversing the linked list
  
class LinkedList {
  
    static Node head;
  
    static class Node {
  
        int data;
        Node next;
  
        Node(int d) {
            data = d;
            next = null;
        }
    }
  
    /* Function to reverse the linked list */
    Node reverse(Node node) {
        Node prev = null;
        Node current = node;
        Node next = null;
        while (current != null) {
            next = current.next;
            current.next = prev;
            prev = current;
            current = next;
        }
        node = prev;
        return node;
    }
  
    // prints content of double linked list
    void printList(Node node) {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }
  
    public static void main(String[] args) {
        LinkedList list = new LinkedList();
        list.head = new Node(85);
        list.head.next = new Node(15);
        list.head.next.next = new Node(4);
        list.head.next.next.next = new Node(20);
          
        System.out.println("Given Linked list");
        list.printList(head);
        head = list.reverse(head);
        System.out.println("");
        System.out.println("Reversed linked list ");
        list.printList(head);
    }
}
  
  
// This code has been contributed by Mayank Jaiswal

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Python

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# Python program to reverse a linked list 
# Time Complexity : O(n)
# Space Complexity : O(1)
  
# Node class 
class Node:
  
    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None
  
class LinkedList:
  
    # Function to initialize head
    def __init__(self):
        self.head = None
  
    # Function to reverse the linked list
    def reverse(self):
        prev = None
        current = self.head
        while(current is not None):
            next = current.next
            current.next = prev
            prev = current
            current = next
        self.head = prev
          
    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node
  
    # Utility function to print the linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next
  
  
# Driver program to test above functions
llist = LinkedList()
llist.push(20)
llist.push(4)
llist.push(15)
llist.push(85)
  
print "Given Linked List"
llist.printList()
llist.reverse()
print "\nReversed Linked List"
llist.printList()
  
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

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Given linked list
85 15 4 20 
Reversed Linked list 
20 4 15 85 

Time Complexity: O(n)
Space Complexity: O(1)




Recursive Method:

   1) Divide the list in two parts - first node and rest of the linked list.
   2) Call reverse for the rest of the linked list.
   3) Link rest to first.
   4) Fix head pointer

Linked List Rverse

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void recursiveReverse(struct Node** head_ref)
{
    struct Node* first;
    struct Node* rest;
       
    /* empty list */
    if (*head_ref == NULL)
       return;   
  
    /* suppose first = {1, 2, 3}, rest = {2, 3} */
    first = *head_ref;  
    rest  = first->next;
  
    /* List has only one node */
    if (rest == NULL)
       return;   
  
    /* reverse the rest list and put the first element at the end */
    recursiveReverse(&rest);
    first->next->next  = first;  
      
    /* tricky step -- see the diagram */
    first->next  = NULL;          
  
    /* fix the head pointer */
    *head_ref = rest;              
}

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Time Complexity: O(n)
Space Complexity: O(1)



A Simpler and Tail Recursive Method
Below is C++ implementation of this method.

C++

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// A simple and tail recursive C++ program to reverse
// a linked list
#include<bits/stdc++.h>
using namespace std;
  
struct Node
{
    int data;
    struct Node *next;
};
  
void reverseUtil(Node *curr, Node *prev, Node **head);
  
// This function mainly calls reverseUtil()
// with prev as NULL
void reverse(Node **head)
{
    if (!head)
        return;
    reverseUtil(*head, NULL, head);
}
  
// A simple and tail recursive function to reverse
// a linked list.  prev is passed as NULL initially.
void reverseUtil(Node *curr, Node *prev, Node **head)
{
    /* If last node mark it head*/
    if (!curr->next)
    {
        *head = curr;
  
        /* Update next to prev node */
        curr->next = prev;
        return;
    }
  
    /* Save curr->next node for recursive call */
    Node *next = curr->next;
  
    /* and update next ..*/
    curr->next = prev;
  
    reverseUtil(next, curr, head);
}
  
// A utility function to create a new node
Node *newNode(int key)
{
    Node *temp = new Node;
    temp->data = key;
    temp->next = NULL;
    return temp;
}
  
// A utility function to print a linked list
void printlist(Node *head)
{
    while(head != NULL)
    {
        cout << head->data << " ";
        head = head->next;
    }
    cout << endl;
}
  
// Driver program to test above functions
int main()
{
    Node *head1 = newNode(1);
    head1->next = newNode(2);
    head1->next->next = newNode(3);
    head1->next->next->next = newNode(4);
    head1->next->next->next->next = newNode(5);
    head1->next->next->next->next->next = newNode(6);
    head1->next->next->next->next->next->next = newNode(7);
    head1->next->next->next->next->next->next->next = newNode(8);
    cout << "Given linked list\n";
    printlist(head1);
    reverse(&head1);
    cout << "\nReversed linked list\n";
    printlist(head1);
    return 0;
}

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Java

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// Java program for reversing the Linked list
  
class LinkedList {
  
    static Node head;
  
    static class Node {
  
        int data;
        Node next;
  
        Node(int d) {
            data = d;
            next = null;
        }
    }
  
    // A simple and tail recursive function to reverse
    // a linked list.  prev is passed as NULL initially.
    Node reverseUtil(Node curr, Node prev) {
  
        /* If last node mark it head*/
        if (curr.next == null) {
            head = curr;
  
            /* Update next to prev node */
            curr.next = prev;
              
            return head;
        }
  
        /* Save curr->next node for recursive call */
        Node next1 = curr.next;
  
        /* and update next ..*/
        curr.next = prev;
  
        reverseUtil(next1, curr);
        return head;
    }
  
    // prints content of double linked list
    void printList(Node node) {
        while (node != null) {
            System.out.print(node.data + " ");
            node = node.next;
        }
    }
  
    public static void main(String[] args) {
        LinkedList list = new LinkedList();
        list.head = new Node(1);
        list.head.next = new Node(2);
        list.head.next.next = new Node(3);
        list.head.next.next.next = new Node(4);
        list.head.next.next.next.next = new Node(5);
        list.head.next.next.next.next.next = new Node(6);
        list.head.next.next.next.next.next.next = new Node(7);
        list.head.next.next.next.next.next.next.next = new Node(8);
  
        System.out.println("Original Linked list ");
        list.printList(head);
        Node res = list.reverseUtil(head, null);
        System.out.println("");
        System.out.println("");
        System.out.println("Reversed linked list ");
        list.printList(res);
    }
}
  
  
// This code has been contributed by Mayank Jaiswal

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Python

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# Simple and tail recursive Python program to 
# reverse a linked list
  
# Node class 
class Node:
  
    # Constructor to initialize the node object
    def __init__(self, data):
        self.data = data
        self.next = None
  
class LinkedList:
  
    # Function to initialize head
    def __init__(self):
        self.head = None
  
  
    def reverseUtil(self, curr, prev):
          
        # If last node mark it head
        if curr.next is None :
            self.head = curr 
              
            # Update next to prev node
            curr.next = prev
            return 
          
        # Save curr.next node for recursive call
        next = curr.next
  
        # And update next 
        curr.next = prev
      
        self.reverseUtil(next, curr) 
  
  
    # This function mainly calls reverseUtil()
    # with previous as None
    def reverse(self):
        if self.head is None:
            return 
        self.reverseUtil(self.head, None)
  
  
    # Function to insert a new node at the beginning
    def push(self, new_data):
        new_node = Node(new_data)
        new_node.next = self.head
        self.head = new_node
  
    # Utility function to print the linked LinkedList
    def printList(self):
        temp = self.head
        while(temp):
            print temp.data,
            temp = temp.next
  
  
# Driver program
llist = LinkedList()
llist.push(8)
llist.push(7)
llist.push(6)
llist.push(5)
llist.push(4)
llist.push(3)
llist.push(2)
llist.push(1)
  
print "Given linked list"
llist.printList()
  
llist.reverse()
  
print "\nReverse linked list"
llist.printList()
  
# This code is contributed by Nikhil Kumar Singh(nickzuck_007)

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Output:

Given linked list
1 2 3 4 5 6 7 8

Reversed linked list
8 7 6 5 4 3 2 1

Thanks to Gaurav Ahirwar for suggesting this solution.

Recursively Reversing a linked list (A simple implementation)
Iteratively Reverse a linked list using only 2 pointers (An Interesting Method)

References:
http://cslibrary.stanford.edu/105/LinkedListProblems.pdf



My Personal Notes arrow_drop_up

Improved By : AshishKumar22