Reverse alternate K nodes in a Singly Linked List | GeeksforGeeks

Given a linked list, write a function to reverse every alternate k nodes (where k is an input to the function) in an efficient way. Give the complexity of your algorithm.

Method 1 (Process 2k nodes and recursively call for rest of the list)

struct node *kAltReverse(struct node *head, int k)

{

    struct node* current = head;

    struct node* next;

    struct node* prev = NULL;

    int count = 0;  

 

    /*1) reverse first k nodes of the linked list */

    while (current != NULL && count < k)

    {

       next  = current->next;

       current->next = prev;

       prev = current;

       current = next;

       count++;

    }

   

    /* 2) Now head points to the kth node.  So change next

       of head to (k+1)th node*/

    if(head != NULL)

      head->next = current;  

 

    /* 3) We do not want to reverse next k nodes. So move the current

        pointer to skip next k nodes */

    count = 0;

    while(count < k-1 && current != NULL )

    {

      current = current->next;

      count++;

    }

 

    /* 4) Recursively call for the list starting from current->next.

       And make rest of the list as next of first node */

    if(current !=  NULL)

       current->next = kAltReverse(current->next, k);

 

    /* 5) prev is new head of the input list */

    return prev;

}

Method 2 (Process k nodes and recursively call for rest of the list) 
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