Robinhood Senior Software Engineer Interview Experience — Passed the Tech Rounds, Failed the EM Round, Redid Team Match

I interviewed for Senior Software Engineer on the Competitor Insights team. I passed the technical rounds, but failed the EM round because I lacked frontend experience. The recruiter agreed to let me redo team match, so I talked to the Experimentation team as well.

Round 1: Hiring Manager
We covered the usual collaboration and communication questions, and I was also asked a lot of questions about frontend optimization.

Round 2: System Design
Design an order execution system where users can place market orders and limit orders, and can also cancel them. Specific questions included:

• How orders get executed, how order status transitions, and how to handle failures
• How to track user balances
• How to scale the system to support millions of orders

Round 3: Coding
The Fractional Share problem.

If Robinhood has 0 shares of AAPL and then a customer wishes to purchase 1.5 (150) AAPL shares, Robinhood will need to request 2 (200) shares from the exchange and hold on to the remaining 0.5 (50) shares.

If another customer requests to purchase 0.4 shares of AAPL, Robinhood can use its inventory (0.5 shares) instead of going out to the exchange and will have 0.1 shares of AAPL remaining.

If the third customer requests 0.5 shares, Robinhood can fill 0.1 shares out of inventory but will need to go to the exchange for an additional share leaving Robinhood's inventory at 0.6 shares.

If a customer requests a dollar based order, we need to convert it to the relevant number of shares and run through the above steps.

Always ensure the firm has a positive quantity in inventory and has under one share after handling an order. There's no need for us to hold onto whole shares!

Steps:

1. Handle buying fractional shares.
2. Handle selling fractional shares.
3. Ensure inventory is less than 1 after each order. e.g. Customer sells AAPL for 0.75 and then another sells AAPL for 0.50 -- we have 1.25 inventory. We can sell 1 share to the market and keep our inventory small at 0.25.
4. Ensure inventory is always non-negative after each order. e.g. Inventory is 0.2 and the customer buys 0.5 shares: ensure we end up with 0.7 shares in inventory.
5. Always "flatten"! (steps 3+4)

The final 2 digits of every integer is the decimal. e.g. 1000 = 10.00, 20 = 0.20, 100 = 1.

Example scenario:

Input:
// One AAPL buy order for 0.42 shares. AAPL is currently worth $1.
orders: [["AAPL","B","42","100"]]
// Inventory for AAPL is currently 0.99 shares.
inventory: [["AAPL","99"]]

Expected Output:
// The users buys 0.42 shares from inventory, leaving us with 0.57 shares.
[["AAPL","57"]]

Another example scenario:

Input:
// One AAPL buy order for $0.42. AAPL is currently worth $1, so that's 0.42 shares.
orders: [["AAPL","B","$42","100"]]
// Existing AAPL inventory is 0.50 shares.
inventory: [["AAPL","50"]]

Expected Output:
// 0.50 - 0.42 = 0.08 shares leftover.
[["AAPL","8"]]

Team Match 2: Experimentation team HM
They asked a lot of domain-related questions, like A/B testing experience, statistics experience, and data infrastructure.