Process Queries on Cart

Problem Overview

• Manage a cart by processing add/remove queries on an integer list.
• Input: initial items array and a query array; Output: final cart array after all queries; n, q ≤ 2e5; IDs up to 1e9; queries are nonzero.
• Positive query appends the ID to the cart; negative query removes the first matching ID; final cart is guaranteed non-empty.
• Real-world context: e-commerce cart prototype.
• From Amazon interviews; a coding interview problem and interview question.