Amazon SDE-1 — DSA + LLD Onsite (Learnings from a Messy LLD)

Round 1 — DSA + LP (Onsite)

DSA 1: Minimum cost path in a grid of 0s and 1s. You can flip a 1 to 0 at cost 1; find min cost from (0,0) to (n-1,m-1). Solved it (0-1 BFS / Dijkstra approach).

DSA 2: A problem similar to House Robber II (circular), though the statement was framed differently. Solved it.

LP: Leadership Principle questions went well.

Round 2 — LLD (Onsite)

This is where I learned a valuable lesson.

Problem: Design devices with different capabilities:

• Every device can tell if it's plugged in and report charging percentage
• Some devices have a Display (show a message)
• Some have a Speaker (play/speak a message)
• Different devices support different combinations (e.g., a Tablet has device + display + speaker)

What I did:

• Created a Device interface with isPluggedIn() and getChargingPercentage()
• Created a Display interface with showMessage()
• Created a Speaker interface with a speak method
• For the Tablet, I implemented all three interfaces

Where it went wrong:

• My design became messy. I ended up duplicating functionality and the Tablet implementation felt like it was implementing the same thing multiple times.
• I realized mid-interview that I wasn't applying interface segregation cleanly, and couldn't bring the implementation to the level expected.

What I should have done:

• Kept the interfaces cleanly segregated (each capability = one focused interface)
• Used composition properly — the Tablet composes the capabilities rather than muddling them
• Thought about extensibility upfront (adding a new capability without touching existing code)

Lessons Learned

1. Practice LLD, not just DSA. SDE-1 at Amazon includes a real LLD round, and clean OOP design under time pressure is a separate skill.
2. Interface Segregation Principle matters — don't create fat interfaces or force classes to implement things they don't need.
3. Think before coding in LLD — spend the first few minutes designing the class/interface structure. A messy start compounds.
4. Composition over inheritance — especially when objects have varying combinations of capabilities.

Tips for Amazon SDE-1

• Two DSA problems + LP in one round is common. Practice medium DSA with clean code.
• The LLD round is real — practice classic LLD problems (parking lot, elevator, this device-capabilities type).
• Know your SOLID principles, especially Interface Segregation and Open/Closed.
• LPs are asked in every round — prepare STAR stories.