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

This article is contributed by **Chhavi**. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.