Recently I helped several classmates do a few shows Citadel OA, the overall feeling is very unified: time is tight + the questions are not difficult but have many details. What really hinders people is not the algorithm, but the modeling ability + the speed of writing code. This article directly breaks down the latest round of OA situation + high-frequency questions for each position.
How long does Citadel OA take?
Based on candidate feedback, OA typically lasts 75–90 minutes, depending on the role:
- Software engineering: 2 programming questions + quantitative/logical questions.
- Quantitative research: A mix of probability, programming, and domain-specific problems.
- Finance/data positions: Quantitative questions + logical challenges with a financial background.
Time management is crucial. A common strategy is to spend 60–70% of your time on what you do best and then use the rest of the time to attack the hard part.
Citadel HackerRank Question Exclusive Sharing
Citadel SWE HackerRank Question
Issue 1: Palindrome Cipher Conversion
Topic description: You are given a password string consisting of lowercase letters. You need to convert it to a new password of the same length, which must meet two conditions:
- The new password must be a palindrome (the same as the original and reverse).
- Every k characters must be the same (that is, characters whose positions are a multiple of k apart must be the same).
Returns the minimum number of character modifications required to make the original password meet the conditions.
Enter:
- String s (1 ≤ |s| ≤ 10⁵), containing only lowercase English letters.
- Integer k (1 ≤ k ≤ |s|).
Output: An integer representing the minimum number of modifications required.
Example:
Plaintext
Input: s = "abcda" k = 2 Output: 2 Explanation: - To be a palindrome, positions (0,4) and (1,3) must be the same. - To satisfy the condition k=2, positions (0,2,4) must be the same and positions (1,3) must be the same. At least 2 revisions are required.
Problem 2: Process scheduling without consecutive repetitions
Topic description: You are given n processes and need to schedule them into n_intervals time slots. Only one process must be scheduled per time slot. The same process cannot be scheduled in two consecutive time slots.
Returns the total number of valid schedules.
Enter:
- Two integers: n (1 ≤ n ≤ 20) and n_intervals (1 ≤ n_intervals ≤ 50).
Output: An integer indicating the number of valid scheduling plans.
Example 1:
Input: n = 2 n_intervals = 3 Output: 6 Explanation: There are two processes (A, B). Valid schedules of length 3: ABA, BAB, ABB, BAA, AAB, BBA → 6 types in total.
Example 2:
Input: n = 3 n_intervals = 2 Output: 6 Explanation: There are three processes (A, B, C). Valid schedules with length 2 and no consecutive repetitions: AB, AC, BA, BC, CA, CB → 6 types in total.
Citadel Quantitative Trading HackerRank Question
The following are common probability and expected value issues in quantitative trading positions:
- Frog Jump Problem The frog jumps from Ground → Rock → Grass.
- P(Ground → Rock) = 1/3
- P(Rock → Grass) = 1/4 Calculation: The expected number of jumps required to escape (E[num of jumps to escape]).
- Tournament Best of 5 Two teams play until one team wins 3 games (best of 5). Calculate:The expected number of games played (E[num of games played]).
- Socks Pairing Probability 10 socks: 5 red, 5 white. Calculate:The probability of forming two pairs after drawing 4 socks.
- Expected number of adjacent pairs with the same hair color (Adjacent Pairs with Same Hair Color) 4 had red hair and 8 had black hair. Calculate:The expected number of pairs of adjacent pairs w/ same color hair (E[num of adjacent pairs w/ same color hair]).
- Bus Lateness Problem
- The probability of the red train being late is 3/4
- The probability that the blue bus will be late is 1/3. A person randomly takes the bus. It is known that the probability of taking the red car is 1/2 and the person is late, calculateThe ratio of red cars to blue cars.
- Tourists on Islands Three tourists independently and randomly selected one of the three islands. Calculate:Expected value of the median number of visitors to the three islands.
- Neighborhood Walk Time A community of 25 people has a mean walking time of 80 minutes and a standard deviation of 10 minutes. Calculation: Standard deviation of total walking time for the entire neighborhood.
Citadel NXT HackerRank Question
Problem: Implementing an efficient order book matching engine
Allowed time: 45 minutes
Target: Implements a simplified core order book functionality that efficiently receives and processes buy and sell orders.
Data structure design requirements:
- Use max-heap to store all buy orders (Bids), with order price priority (highest bid price first).
- Use min-heap to store all sell orders (Asks), with order price priority (lowest ask price first).
Matching logic:
- Buy order matching: If the price of the new buy order ≥ the current lowest sell order price, a transaction occurs.
- Sell order matching: If the price of the new sell order ≤ the current highest buy order price, a transaction occurs.
- After a transaction occurs, the matched order is removed from the pile and the remaining quantity is processed.
Functional requirements:
- Write a central function to handle new orders.
- Input: order type (buy/sell), price, quantity.
- Output:
- Total trading volume
- The status of the order book after the trade (best bid and best ask)
One-stop job search assistance | OA + full interview process support
Recently, many students are preparing for OA/interviews in large companies. Due to tight time and high pressure, it is indeed easy to overturn in key links.Programhelp Mainly providing auxiliary support related to OA and interviews. I am familiar with platforms such as HackerRank, CodeSignal, and Niuke, and I have compiled many high-frequency questions and ideas. During this period of time, we have indeed helped many students successfully land in large companies and quantitative positions, but many of them are actually lacking in a little bit of rhythm and thinking on the spot. If you are a little unsure about your own preparation, or you are always stuck in a certain link, you can also come and have a chat to see how to optimize it more appropriately.
