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Program to Reverse only First N Elements of a Linked List



Program to Reverse only First N Elements of a Linked List

    #include <stdio.h>

    #include <stdlib.h>

    struct node

    {

        int number;

        struct node *next;

    };

    void create(struct node **);

    void reverse(struct node **, int);

    void release(struct node **);

    void display(struct node *);

    int main()

    {

        struct node *p = NULL;

        int n;

        printf("Enter data into the list\n");

        create(&p);

        printf("Displaying the nodes in the list:\n");

        display(p);

        printf("Enter the number N to reverse first N node: ");

        scanf("%d", &n);

        printf("Reversing the list...\n");

        if (n > 1)

        {

            reverse(&p, n - 2);

        }

        printf("Displaying the reversed list:\n");

        display(p);

        release(&p);

        return 0;

    }

    void reverse(struct node **head, int n)

    {

        struct node *p, *q, *r, *rear;

        p = q = r = *head;

        if (n == 0)

        {

            q = q->next;

            p->next = q->next;

            q->next = p;

            *head = q;

        }

        else

        {

            p = p->next->next;

            q = q->next;

            r->next = NULL;

            rear = r;

            q->next = r;

            while (n > 0 && p != NULL)

            {

                r = q;

                q = p;

                p = p->next;

                q->next = r;

                n--;

            }

            *head = q;

            rear->next = p;

        }

    }

    void create(struct node **head)

    {

        int c, ch;

        struct node *temp, *rear;

        do

        {

            printf("Enter number: ");

            scanf("%d", &c);

            temp = (struct node *)malloc(sizeof(struct node));

            temp->number = c;

            temp->next = NULL;

            if (*head == NULL)

            {

                *head = temp;

            }

            else

            {

                rear->next = temp;

            }

            rear = temp;

            printf("Do you wish to continue [1/0]: ");

            scanf("%d", &ch);

        } while (ch != 0);

        printf("\n");

    }

    void display(struct node *p)

    {

        while (p != NULL)

        {

            printf("%d\t", p->number);

            p = p->next;

        }

        printf("\n");

    }

    void release(struct node **head)

    {

        struct node *temp = *head;

        *head = (*head)->next;

        while ((*head) != NULL)

        {

            free(temp);

            temp = *head;

            (*head) = (*head)->next;

        }

    }