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Tuesday, 11 July 2017 12:34

Reverse a linked list

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Given pointer to the head node of a linked list, the task is to reverse the linked list.

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
Iterate trough the linked list. In loop, change next to prev, prev to current and current to next.

Implementation of Iterative Method

#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;
    while (current != NULL)
    {
        next  = current->next; 
        current->next = prev;  
        prev = current;
        current = next;
    }
    *head_ref = prev;
}
 
/* Function to push a node */
void push(struct Node** head_ref, int new_data)
{
    /* allocate node */
    struct Node* new_node =
            (struct Node*) malloc(sizeof(struct Node));
            
    /* put in the data  */
    new_node->data  = new_data;
                
    /* link the old list off the new node */
    new_node->next = (*head_ref);   
        
    /* move the head to point to the new node */
    (*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();

 

}
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

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;             
}

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



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

// 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;

 

}
Output:
Given linked list
1 2 3 4 5 6 7 8

Reversed linked list
8 7 6 5 4 3 2 1
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Anish Sir

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