Write SQL and merge linked lists

The technical interview included two coding-style tasks: a SQL analytics query and merging two sorted linked lists.

Constraints & Assumptions

• For SQL, return one row per customer, including customers with no orders.
• Report total_orders, total_amount, latest_order_date, and latest_order_amount.
• Use a nested query, CTE, or window function to identify each customer's most recent order.
• For linked lists, reuse existing nodes and return the merged head.

Clarifying Questions to Ask

• If a customer has multiple orders on the same latest date, how should ties be handled?
• Should returned orders affect total order amount?
• Should total_amount be 0 or NULL for customers with no orders?
• Are linked-list values sorted in non-decreasing order?

Part 1 - Write The SQL Query

Given customers, orders, and returns, write a query returning customer-level order metrics and latest order amount.

What This Part Should Cover

• Aggregate orders by customer.
• Use ROW_NUMBER() or similar to find the latest order per customer.
• LEFT JOIN from customers so customers without orders remain.
• COALESCE for zero counts and amounts if desired.

Part 2 - Merge Two Sorted Linked Lists

Given heads of two sorted singly linked lists, merge them into one sorted list by reusing nodes.

What This Part Should Cover

• Dummy head and tail pointer.
• Compare current node values and append the smaller node.
• Attach the remaining suffix.
• Complexity and edge cases.