Warehouse Stock - Process Inventory Events and Answer Range Queries

Question

## Problem

A warehouse processes a stream of inventory events: `(sku, event_type, quantity)` where `event_type` is `"restock"` or `"sale"`. Implement an inventory system that:

1. Applies events in order (restock adds, sale subtracts -- stock never goes below 0)
2. Answers: "How many SKUs currently have stock between `lo` and `hi` units (inclusive)?"

```python
class Warehouse:
    def __init__(self): ...
    def process(self, sku: str, event_type: str, qty: int) -> None:
    def count_in_range(self, lo: int, hi: int) -> int:
```

## Example

```
w = Warehouse()
w.process("A", "restock", 10)
w.process("B", "restock", 5)
w.process("A", "sale", 3)
w.count_in_range(5, 10)  -> 2  (A=7, B=5)
w.process("B", "sale", 5)
w.count_in_range(5, 10)  -> 1  (A=7; B=0 out of range)
```

## Follow-ups

1. How would you make `count_in_range` faster than O(n) using a sorted structure or Fenwick tree?
2. How do you handle concurrent restocks and sales in a multi-threaded environment?
3. Add `low_stock_alert(threshold)` that returns all SKUs with stock below `threshold`.
4. Extend to multiple warehouses and answer range queries across all warehouses combined.

Warehouse Stock - Process Inventory Events and Answer Range Queries

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