Delete N Nodes After M Nodes of a Linked List

Delete N Nodes After M Nodes of a Linked List

1. What is this problem about?

The Delete N Nodes After M Nodes of a Linked List interview question asks you to modify a linked list by skipping a certain number of nodes and then deleting a specific number of subsequent nodes. You repeat this skip-and-delete process until you reach the end of the list. It’s a practical exercise in pointer manipulation and linked list traversal.

2. Why is this asked in interviews?

Microsoft often uses the Delete N Nodes After M Nodes of a Linked List coding problem to assess a candidate's basic data structure proficiency. It tests your ability to navigate a Linked List interview pattern without losing track of pointers, handling edge cases like empty lists, or scenarios where the number of nodes to delete exceeds the remaining length of the list.

3. Algorithmic pattern used

This problem is a pure Two-Pointer / Traversal simulation. You maintain a current pointer to traverse the list. You use a counter or a nested loop to move forward M times, then another loop to skip/delete the next N nodes by adjusting the next pointer of the $M^{th}$ node.

4. Example explanation

Suppose you have a list: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8, with $M=2$ and $N=3$.

1. Start at 1. Move $M=2$ steps. Current is at node 2.
2. Now skip $N=3$ nodes (3, 4, 5).
3. Connect node 2's next pointer to node 6.
4. The list is now 1 -> 2 -> 6 -> 7 -> 8.
5. Repeat: Move $M=2$ steps from node 6. Current is at 7.
6. Skip $N=3$ nodes. Since there is only one node (8) left, node 7's next becomes null. Final list: 1 -> 2 -> 6 -> 7.

5. Common mistakes candidates make

• Null Pointer Exceptions: Not checking if current or current.next is null before trying to move forward or skip.
• Off-by-one errors: Moving one step too many or too few during the "M skip" phase.
• Losing the list tail: Failing to set the last kept node's next pointer to null if the deletion goes to the end of the list.

6. Interview preparation tip

For any linked list problem involving "groups" of nodes, try to visualize the pointer jumps. Drawing the list on a whiteboard and tracing the "next" pointer updates step-by-step is the best way to avoid logic errors during the implementation phase.