C Exercises: Reverse a singly linked list in blocks of size k

C Singly Linked List : Exercise-36 with Solution

Write a C program to reverse a singly linked list starting at the first position in blocks of size k.

Sample Solution:

C Code:

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

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

void push_node(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;
}

struct Node* reverse_k_nodes(struct Node* head, int k) {
    struct Node* current = head;
    struct Node* next = NULL;
    struct Node* prev = NULL;
    int count = 0;

    /* Reverse first k nodes of the linked list */
    while (current != NULL && count < k) {
        next = current->next;
        current->next = prev;
        prev = current;
        current = next;
        count++;
    }

    /* Call recursively for the remaining nodes and link the two lists */
    if (next != NULL) {
        head->next = reverse_k_nodes(next, k);
    }

    /* prev is now the head of the reversed sub-list */
    return prev;
}

void printList(struct Node* node) {
    while (node != NULL) {
        printf("%d ", node->data);
        node = node->next;
    }
}

int main() {
    struct Node* head = NULL;
    push_node(&head, 8);
    push_node(&head, 7);
    push_node(&head, 6);
    push_node(&head, 5);
    push_node(&head, 4);
    push_node(&head, 3);
    push_node(&head, 2);
    push_node(&head, 1);

    printf("Given linked list: \n");
    printList(head);
    head = reverse_k_nodes(head, 3);
    printf("\nReverse the first 3 nodes of the said Linked list: \n");
    printList(head);
    head = reverse_k_nodes(head, 5);
    printf("\nReverse the first 5 nodes of the said Linked list: \n");
    printList(head);
    return 0;
}

Sample Output:

Given linked list: 
1 2 3 4 5 6 7 8 
Reverse the first 3 nodes of the said Linked list: 
3 2 1 6 5 4 8 7 
Reverse the first 5 nodes of the said Linked list: 
5 6 1 2 3 7 8 4  

Flowchart :

C Programming Code Editor:

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