ronith
7/23/2018 - 7:17 AM

Rotate a Linked List

Given a singly linked list, rotate the linked list counter-clockwise by k nodes. Where k is a given positive integer. For example, if the given linked list is 10->20->30->40->50->60 and k is 4, the list should be modified to 50->60->10->20->30->40. Assume that k is smaller than the count of nodes in linked list.

Algorithm: To rotate the linked list, we need to change next of kth node to NULL, next of last node to previous head node, and finally change head to (k+1)th node. So we need to get hold of three nodes: kth node, (k+1)th node and last node. Traverse the list from beginning and stop at kth node. Store pointer to kth node. We can get (k+1)th node using kthNode->next. Keep traversing till end and store pointer to last node also. Finally, change pointers as stated above.

Rotate a Linked List.cpp
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// https://www.geeksforgeeks.org/rotate-a-linked-list/
#include <iostream>
#include <list>
using namespace std;

struct linked_list {
    int data;
    struct linked_list *next;
};
typedef struct linked_list node;

void print (node *n) {
    while (n) {
        cout<< n->data<< "->";
        n = n->next;
    }
    cout<< "NULL";
    return;
}

void insert(node **headref, int n) {
    node* last = *headref;
    node *temp=new node;

    temp->data = n;
    temp->next=NULL;
    if (last == NULL) {
        *headref = temp;
        return;
    }
    while (last->next)
        last = last->next;
    last->next = temp;
    return;
}

void rotate(node **headref, int k) {
    node *tempk = *headref;
    while (k>1){
        tempk = tempk->next;
        k--;
    }
    if (tempk == NULL)
        return;

    node *last = tempk;
    while (last->next)
        last = last->next;
    last->next = *headref;
    *headref = tempk->next;
    tempk->next = NULL;
}

int main() {
    int n;
    node *head=NULL;
    while (true) {
        cout<< "Enter the number: ";
        cin>>n;
        if (n==-9)
            break;
        else
            insert(&head,n);
    }
    cout<< "Enter k: ";
    cin>>n;
    cout<< "Before rotating: ";
    print(head);
    rotate(&head, n);
    cout<< "\nAfter rotating: ";
    print(head);
}