We are given a linked list (like [2 → 4 → 7 → 8])

and a number k.

We must return the k-th node from the end.

Example:

• list = [2,4,7,8], k = 1 → answer = 8 (last node)
• list = [2,4,7,8], k = 2 → answer = 7 (second last node)
• list = [2,4,7,8], k = 3 → answer = 4 (third last node)

🎯 One-Pass Solution (Two Runners Trick)

Key Hack / Initial Trick 🧠

👉 Start Runner A k steps ahead of Runner B.

Why?

Because then when Runner A reaches the end of the list,

Runner B will automatically be k steps behind → exactly at the node we want. ✅

Step-by-Step

1. Put Runner A and Runner B at the head (start of the list).

2. Move Runner A forward k steps.

   • Now Runner A is k nodes ahead of Runner B.

3. Move both runners forward together, one step at a time.

4. When Runner A reaches the end →

   Runner B is standing at the k-th node from the end. 🎉