Linked list traversal after reversing the linked list

Question

I've been trying to run this linkedListTraversal() function after reversing the linked list using the function reverseLinkedList(). I know that I'm applying the correct logic in the reverseLinkedList() function. But for some reason i'm getting the output as something like this

Here's my code

#include <stdio.h>
#include <stdlib.h>

struct Node
{  
   int data;
   struct Node *next;
};
// struct Node *head;

void linkedListTraversal(struct Node *ptr) 
{
// struct Node *ptr = head;
   while (ptr != NULL)
   {
       printf("Element: %d\n", ptr->data);
       ptr = ptr->next;
   }
}

void reverseLinkedList(struct Node *head)
{
    struct Node *prevNode = NULL;
    struct Node *currNode = head;
    struct Node *nextNode;

    while (currNode != NULL)
    {
       nextNode = currNode->next;
       currNode->next = prevNode;
       prevNode = currNode;
       currNode = nextNode;
    }

    head = prevNode;
}

int main()
{
    struct Node *head;
    struct Node *second;
    struct Node *third;

    head = (struct Node *)malloc(sizeof(struct Node));
    second = (struct Node *)malloc(sizeof(struct Node));
    third = (struct Node *)malloc(sizeof(struct Node));

    head->data = 1;
    head->next = second;

    second->data = 2;
    second->next = third;

    third->data = 3;
    third->next = NULL;

    printf("Linked list before reversal: \n");
    linkedListTraversal(head);

    reverseLinkedList(head);

    printf("Linked list after reversal: \n");
    linkedListTraversal(head);
    return 0;
 }

pass the address to `reverseLinkedList(&head);` & update head value to reflect. OR return new `head` value from `reverseLinkedList()` to update the `head` in caller function. — जलजनक, Feb 24 '22 at 20:09
As an aside - you have memory leaks (you never free your nodes). — BitTickler, Feb 24 '22 at 20:33

Craig Estey · Answer 1 · 2022-02-24T21:41:06.677

You're not passing back the updated head to main.

The easiest way is to return the head:

struct Node *reverseLinkedList(struct Node *head);

And have main do:

head = reverseLinkedList(head);

Side note: No need to cast the results of malloc: Do I cast the result of malloc? Here's a more idiomatic way to allocate (e.g.):

head = malloc(sizeof(*head));

Here's the full code:

#include <stdio.h>
#include <stdlib.h>

struct Node {
    int data;
    struct Node *next;
};

// struct Node *head;

void
linkedListTraversal(struct Node *ptr)
{
// struct Node *ptr = head;
    while (ptr != NULL) {
        printf("Element: %d\n", ptr->data);
        ptr = ptr->next;
    }
}

struct Node *
reverseLinkedList(struct Node *head)
{
    struct Node *prevNode = NULL;
    struct Node *currNode = head;
    struct Node *nextNode;

    while (currNode != NULL) {
        nextNode = currNode->next;
        currNode->next = prevNode;
        prevNode = currNode;
        currNode = nextNode;
    }

    head = prevNode;

    return head;
}

int
main(void)
{
    struct Node *head;
    struct Node *second;
    struct Node *third;

    head = malloc(sizeof(*head));
    second = malloc(sizeof(*second));
    third = malloc(sizeof(*third));

    head->data = 1;
    head->next = second;

    second->data = 2;
    second->next = third;

    third->data = 3;
    third->next = NULL;

    printf("Linked list before reversal: \n");
    linkedListTraversal(head);

    head = reverseLinkedList(head);

    printf("Linked list after reversal: \n");
    linkedListTraversal(head);

    return 0;
}

Linked list traversal after reversing the linked list

1 Answers1