Reverse every k nodes of a linked list

Question

I am preparing for a technical interview and I am stuck at writing this program to reverse every k nodes of a linked list.

For example

1->2->3         


        

Answer 1

(I'm assuming this is a singly linked list.) You can keep a temporary pointer (lets call it nodek) and advance it k times in a while loop. This will take O(k). Now you have a pointer to the start of the list and to the last element of the sublist. To reverse here, you remove from head which is O(1) and add to nodek which is O(1). Do this for all elements, so this is O(k) again. Now update root to nodek, and run the while loop on nodek again (to get the new position of nodek) and repeat this whole process again. Remember to do any error checking along the way. This solution will run at O(n) with only O(1) extra space.