C Exercises: Reverse alternate k nodes of a singly linked list

Last update on March 19 2025 12:36:46 (UTC/GMT +8 hours)

41. Alternate K-Node Reversal Challenges

Write a C program to reverse alternate k nodes of a given singly linked list.

Sample Solution:

C Code:

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

// Definition for singly-linked list
struct Node {
    int data;
    struct Node* next;
};

// Function to create a new node in the linked list
struct Node* newNode(int data) {
    struct Node* node = (struct Node*) malloc(sizeof(struct Node)); // Allocate memory for a new node
    node->data = data; // Assign data to the new node
    node->next = NULL; // Initialize the next pointer as NULL
    return node; // Return the newly created node
}

// Function to reverse alternate K nodes in a linked list
struct Node* reverse_Alt_K_Nodes(struct Node* head, int k) {
    struct Node* current = head;
    struct Node* prev = NULL;
    struct Node* next = NULL;
    int count = 0;

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

    /* head points to k+1th node */
    if (head != NULL) {
        head->next = current;
    }

    /* skip next k nodes */
    count = 0;
    while (count < k - 1 && current != NULL) {
        current = current->next;
        count++;
    }

    /* recursively call the function on the remaining list */
    if (current != NULL) {
        current->next = reverse_Alt_K_Nodes(current->next, k);
    }

    /* prev now points to the head of the reversed k nodes */
    return prev;
}

// Function to print the linked list
void printList(struct Node* head) {
    while (head != NULL) {
        printf("%d ", head->data);
        head = head->next;
    }
    printf("\n");
}

// Main function to demonstrate reversing alternate K nodes in a linked list
int main() {
    // Create a sample linked list
    struct Node* head = newNode(1);
    head->next = newNode(2);
    head->next->next = newNode(3);
    head->next->next->next = newNode(4);
    head->next->next->next->next = newNode(5);
    head->next->next->next->next->next = newNode(6);
    head->next->next->next->next->next->next = newNode(7);
    head->next->next->next->next->next->next->next = newNode(8);

    printf("Original List: ");
    printList(head);

    head = reverse_Alt_K_Nodes(head, 2);
    printf("\nReverse alternate k (k=2) nodes of the said singly linked list:\n");
    printList(head);
    head = reverse_Alt_K_Nodes(head, 3);
    printf("\nReverse alternate k (k=3) nodes of the said singly linked list:\n");
    printList(head);
    head = reverse_Alt_K_Nodes(head, 4);
    printf("\nReverse alternate k (k=4) nodes of the said singly linked list:\n");
    printList(head);
    return 0;
}

Sample Output:

Original List: 1 2 3 4 5 6 7 8

Reverse alternate k (k=2) nodes of the said singly linked list:
2 1 3 4 6 5 7 8

Reverse alternate k (k=3) nodes of the said singly linked list:
3 1 2 4 6 5 8 7

Reverse alternate k (k=4) nodes of the said singly linked list:
4 2 1 3 6 5 8 7

Flowchart :

For more Practice: Solve these Related Problems:

Write a C program to reverse alternate groups of k nodes in a linked list and then merge them with the unreversed groups in reverse order.
Write a C program to reverse alternate k nodes recursively and then perform a pairwise swap on the reversed segments.
Write a C program to reverse alternate groups of k nodes only if the sum of the nodes in the group is even.
Write a C program to reverse alternate k nodes in a linked list and then rotate the entire list by k positions.

C Programming Code Editor:

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