Citadel Coding Interview Questions
25 Citadel coding interview problems with full optimal solutions — 8 easy, 12 medium, 5 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Citadel interviewer values, and a FAQ section.
Showing 2 problems of 25
- #23hardfrequently asked
23. Merge K Sorted Lists
Merging K sorted streams is a critical primitive in market data aggregation — combining order flow from K exchanges into a single time-ordered feed. Citadel uses this as a hard problem to test heap fundamentals, divide-and-conquer thinking, and whether you can argue O(n log k) vs O(nk) with precision.
- #42hardfrequently asked
42. Trapping Rain Water
Trapping Rain Water is a high-signal hard problem at Citadel — it has three fundamentally different O(n) solutions (prefix/suffix max arrays, two-pointer, monotonic stack), and the interviewer will ask for all of them. It also tests mathematical reasoning: the water at position i is determined by min(leftMax, rightMax) − height[i].