I interviewed at Citadel (New York, NY) in Jan 2026
Interview
1. Algorithmic clarity
Can you identify the core pattern quickly?
Think: prefix sums, sliding windows, graph traversal, greedy with proof, DP state design.2. Mathematical reasoning
They love problems where correctness requires a proof or invariant.
Example: why a greedy choice works, why a hash remainder trick detects divisibility.
3. Performance instincts
You’re expected to reason about:
Time complexity under constraints
Memory vs speed tradeoffs
Edge cases at scale (10⁶–10⁹ inputs)
This is finance engineering. Latency is money.
Interview questions [1]
Question 1
1. Prefix sum + hashing
Why? O(1) lookups under heavy data flow.
Examples:
Subarray sum divisible by k
Longest equal 0/1 subarray
Detect arbitrage-like patterns
What they care about:
Do you understand why equal remainders imply divisible differences?
I didn't make it past the OA. The OA was automatically sent. The OA was very hard. It was only two questions but it was super hard. Apply at your own risk.