57.c

// Reverse Linked List

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

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

struct Node* createNode(int value) {
    struct Node* newNode = (struct Node*)malloc(sizeof(struct Node));
    newNode->data = value;
    newNode->next = NULL;
    return newNode;
}

struct Node* insertAtEnd(struct Node* head, int value) {
    struct Node* newNode = createNode(value);

    if (head == NULL) return newNode;

    struct Node* temp = head;
    while (temp->next != NULL)
        temp = temp->next;

    temp->next = newNode;
    return head;
}

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

struct Node* reverse(struct Node* head) {
    struct Node* prev = NULL;
    struct Node* curr = head;
    struct Node* next = NULL;

    while (curr != NULL) {
        next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }

    return prev;
}

int main() {
    struct Node* head = NULL;

    head = insertAtEnd(head, 10);
    head = insertAtEnd(head, 20);
    head = insertAtEnd(head, 30);

    printf("Original List:\n");
    display(head);

    head = reverse(head);

    printf("Reversed List:\n");
    display(head);

    return 0;
}