Rotate Linked List block wise

3

Given a Linked List of length n and block length k rotate in circular manner towards right/left each block by a number d. If d is positive rotate towards right else rotate towards left.

Examples:

Input : 1->2->3->4->5->6->7->8->9->NULL, 
        k = 3 
        d = 1
Output : 3->1->2->6->4->5->9->7->8->NULL
Explanation : Here blocks of size 3 are
rotated towards right(as d is positive) 
by 1.
 
Input : 1->2->3->4->5->6->7->8->9->10->
               11->12->13->14->15->NULL, 
        k = 4 
        d = -1
Output : 2->3->4->1->6->7->8->5->10->11
             ->12->9->14->15->13->NULL
Explanation : Here, at the end of linked 
list, remaining nodes are less than k, i.e.
only three nodes are left while k is 4. 
Rotate those 3 nodes also by d.

Prerequisite : Rotate a linked list

The idea is if the absolute alue of d is greater than the value of k, then rotate the link list by d % k times. If d is 0, no need to rotate the linked list at all.

// C/C++ program to rotate a linked list block wise
#include <stdio.h>
#include <stdlib.h>

/* Link list node */
struct Node {
    int data;
    struct Node* next;
};

// Recursive function to rotate one block
struct Node* rotateHelper(struct Node* blockHead,
                          struct Node* blockTail,
                          int d, struct Node** tail,
                          int k)
{
    if (d == 0)
        return blockHead;

    // Rotate Clockwise
    if (d > 0) {
        struct Node* temp = blockHead;
        for (int i = 1; temp->next->next &&
                        i < k - 1; i++)
            temp = temp->next;
        blockTail->next = blockHead;
        *tail = temp;
        return rotateHelper(blockTail, temp,
                            d - 1, tail, k);
    }

    // Rotate anti-Clockwise
    if (d < 0) {
        blockTail->next = blockHead;
        *tail = blockHead;
        return rotateHelper(blockHead->next,
                blockHead, d + 1, tail, k);
    }
}

// Function to rotate the linked list block wise
struct Node* rotateByBlocks(struct Node* head,
                                 int k, int d)
{
    // If length is 0 or 1 return head
    if (!head || !head->next)
        return head;

    // if degree of rotation is 0, return head
    if (d == 0)
        return head;

    struct Node* temp = head, *tail = NULL;

    // Traverse upto last element of this block
    int i;
    for (i = 1; temp->next && i < k; i++)
        temp = temp->next;

    // storing the first node of next block
    struct Node* nextBlock = temp->next;

    // If nodes of this block are less than k.
    // Rotate this block also
    if (i < k)
        head = rotateHelper(head, temp, d % k,
                                    &tail, i);
    else
        head = rotateHelper(head, temp, d % k,
                                    &tail, k);

    // Append the new head of next block to
    // the tail of this block
    tail->next = rotateByBlocks(nextBlock, k,
                                      d % k);

    // return head of updated Linked List
    return head;
}

/* UTILITY FUNCTIONS */
/* Function to push a node */
void push(struct Node** head_ref, int new_data)
{
    struct Node* new_node = new Node;
    new_node->data = new_data;
    new_node->next = (*head_ref);
    (*head_ref) = new_node;
}

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

/* Driver program to test above function*/
int main()
{
    /* Start with the empty list */
    struct Node* head = NULL;

    // create a list 1->2->3->4->5->
    // 6->7->8->9->NULL
    for (int i = 9; i > 0; i -= 1)
        push(&head, i);

    printf("Given linked list \n");
    printList(head);

    // k is block size and d is number of
    // rotations in every block.
    int k = 3, d = 2;
    head = rotateByBlocks(head, k, d);

    printf("\nRotated by blocks Linked list \n");
    printList(head);

    return (0);
}

Output:

Given linked list 
1 2 3 4 5 6 7 8 9 
Rotated by blocks Linked list 
2 3 1 5 6 4 8 9 7 

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