Cart Merge

Problem

A shopper adds items as a guest, then logs in and already has a saved cart. Merge the two. Each cart is a list of {sku, qty} in the order items were added. Rules:

• Same sku in both: quantities add up, capped by max_qty[sku] (default cap 10 if the SKU is not in the map).
• Keep the user cart's order first, then guest-only SKUs in guest order.
• Drop any line whose final quantity is 0.
• SKUs listed in out_of_stock are dropped entirely and returned separately so the UI can show a message.

Return (merged, dropped).

Examples

Example 1

user  = [{A, 2}, {B, 1}]
guest = [{B, 3}, {C, 1}]
max_qty = {B: 3}

→ merged [{A, 2}, {B, 3}, {C, 1}], dropped []. B would be 4 but is capped at 3.

Example 2

user  = [{A, 1}]
guest = [{A, 0}, {D, 2}]
out_of_stock = {D}

→ merged [{A, 1}], dropped [D].

Constraints

• Up to 200 lines per cart; SKUs are strings; 0 <= qty <= 1000.
• Single pass per cart is enough; do not go quadratic on SKU lookup.
• Be explicit about which cart wins on ties in ordering.

What they look for

An ordered map (or dict + list) so order is stable, the cap applied after summing, and the out-of-stock check done before the cap so the dropped list is accurate. The follow-up is usually about what happens if the user's cart changed on another device in between: version numbers or last-write-wins, and where that decision lives.