This is a sample program that when given the head of a linked list, reverses the nodes of the list k at a time, and returns the modified list.

 

k is a positive integer and is less than or equal to the length of the linked list. If the number of nodes is not a multiple of k then left-out nodes, in the end, would remain as it is.

Node reverseK(Node head, int k)

{

int n = 0;

for ( Node cur = head; cur != null; cur=cur.next) {n++};

Node cur = head;

Node prev =null;

Node root = null;

for (int i = 0; i < (n/k) + 1; i++)

{

  Node next = cur;

  For (int j = 0; next != null && j <k-1; j++) { next=next.next;}

  Node Temp = null;

  If (next) {

temp = next.next;

Next.next = null;

  }

  If (temp != null) {

  reverse(ref cur, ref next);

  head =cur;

  next.next = temp;

  }

  if (!root) {root = head};

  while(cur != null && cur.next != null && cur.next != next){ cur = cur.next;};

  if (!prev){

prev = cur;

cur = cur.next;

  } else {  

prev.next=head;

prev = head;

}

  cur=temp;

  next = temp;           

}

return root;

}

 

void reverse(ref node start, ref node end) {

  if (start == null) return;

  if (start.next == null) return;

  Node prev = null;

  Node cur = start;

  Node next = cur.next;

  while (next && cur != end) {

    Cur.next = prev;

    Prev = cur;

    Cur = next;            

    Next = cur.next;

  }

  Cur.next = prev;

  start = cur;

  end = next;

}

 

Sample test cases

List, K                  Result

Null, 0                 Null

[1] 0                    [1]

[1] 1                    [1]

[1] 2                    [1]

[1,2] 1                 [1,2]

[1,2,3], 1             [1,2,3]

[1,2,3], 2             [2,1,3]

[1,2,3], 3             [3,2,1]

[1,2,3,4,5,6], 3                 [3,2,1,6,5,4]

[1,2,3,4,5,6,7], 3              [3,2,1,6,5,4,7]

[1,2,3,4,5,6,7,8], 3             [3,2,1,6,5,4,7,8]