Problem - Reverse Linked List

Given the head of a singly linked list, reverse the list and return the new head. Example: $1 \to 2 \to 3 \to 4 \to 5$ becomes $5 \to 4 \to 3 \to 2 \to 1$.

You're implementing the most fundamental linked list problem. If you can't reverse a list, you can't pass a coding interview.

The iterative solution requires careful pointer manipulation. The recursive solution reveals how the call stack can replace explicit pointers. Constraints: up to $5000$ nodes, values from $-5000$ to $5000$.