I. The Importance of Citadel and OA
In the field of fintech, Citadel stands out as an industry leader with extremely high demands for talent. The Software Development Engineer Intern position at Citadel is highly competitive. Citadel SDE Intern OA, as the first checkpoint in the recruitment process, is of great significance. Through the OA, Citadel can initially screen out candidates with a solid programming foundation and problem-solving abilities, laying the groundwork for subsequent interview stages. For job seekers, successfully passing the OA is the crucial first step to entering the door of Citadel.
II. OA Platform and Basic Settings
The SDE Intern OA of Citadel is usually conducted on the Hackerrank platform. This platform is powerful and supports multiple programming languages, providing candidates with a relatively convenient answering environment. The examination duration is generally 66 minutes (there may be slight differences in different years), and candidates are required to complete two programming questions within the specified time. With a tight schedule and heavy tasks, candidates need to have a thorough understanding of various algorithms and data structures to solve problems efficiently within the limited time.
III. In-depth Analysis of Common Question Types
(I) Algorithmic Question Types
1. Binary Search-related Questions
Binary search frequently appears in Citadel’s OA. For example, in questions like “Maximum Throughput”, candidates need to enumerate all possible minimum values of “throughput” using binary search, and then combine with the given budget conditions to determine whether they are met, using this as the basis for binary search. The key to solving such problems lies in accurately determining the range of binary search and correctly writing the “check” function for judging feasibility. For instance, when the budget is limited, continuously narrow down the value range of “throughput” through binary search until the optimal solution that meets the budget is found.
2. Dynamic Programming (DP) Questions
DP questions are also common in Citadel’s OA and are of considerable difficulty. Classic DP questions like “House Robber” may also appear in modified forms in Citadel’s interviews. The core of such questions is to find the state transition equation. By analyzing and solving different sub-problems, gradually construct the answer to the entire problem. Taking “House Robber” as an example, it is necessary to consider the condition that adjacent houses cannot be robbed simultaneously, and calculate the maximum amount that can be robbed under different numbers of houses through the state transition equation.
3. Graph Algorithm Questions
Graph algorithms, such as Depth-First Search (DFS) and Breadth-First Search (BFS), will also be examined in the OA. For example, given a social network relationship graph and required to calculate the shortest path from one user node to another, the BFS algorithm can be used to solve it. When solving the problem, it is necessary to reasonably construct the data structure of the graph, accurately apply the traversal logic of BFS or DFS, and handle issues such as the access order and status marking of nodes.
(II) Data Structure Question Types
1. Application of Hash Table
Hash tables are often used to solve problems such as searching and counting frequencies. For example, in the question of “finding the length of the longest subarray where the frequency of all elements is k”, first use a hash table to record the frequency of each element, and then combine with the sliding window algorithm. Through the movement of two pointers, quickly find the longest subarray that meets the condition that the element frequency is k. In this process, the hash table can efficiently store and query element frequencies, greatly improving the problem-solving efficiency.
2. Operations on Linked List
Linked list-related questions may examine basic operations such as traversing, inserting, and deleting linked lists, as well as the processing of some special linked lists, such as doubly linked lists. When implementing the “Least Frequently Used (LFU) Cache”, it is necessary to use a doubly linked list and a hash table to achieve efficient cache operations. The doubly linked list is used to maintain the usage order of elements in the cache, and the hash table is used to quickly locate the position of elements in the linked list. Through the combination of the two, the efficiency requirements of the cache system for insertion, deletion, and search operations are met.
(III) Practical Application Scenario Question Types
1. Social Media Network Analysis
Citadel may present some questions related to social media network applications, such as “Friend Recommendation”. The question gives the friendship relationship between users (represented by a two-dimensional array) and requires recommending friends for each user. The solution idea is to determine the recommended objects by analyzing the number of mutual friends between users. If two users are not friends but have the most mutual friends, then recommend one user to the other. If there are multiple users who meet the conditions, recommend the one with the smallest index. Such questions not only examine programming ability but also test candidates’ ability to transform their thinking and apply technology to practical scenarios.
2. Financial Data Processing (Related Examples)
Although not explicitly stated, given Citadel’s financial background, there may be some questions simulating financial data processing. For example, given a series of stock trading data, candidates may be required to calculate the maximum profit within a specific period, or sort and filter financial data to meet certain business rules. This requires candidates to use appropriate algorithms and data structures for programming implementation based on an understanding of financial business logic.
IV. Citadel SDE Intern OA Real Question Sharing
Here are some real questions from Citadel SDE Intern OA to give you a more intuitive understanding.
Question 1: Array Manipulation
Given an integer array nums and an integer k, you need to find the maximum sum of a subarray of length k. For example, if nums = [1, 3, -1, -3, 5, 3, 6, 7] and k = 3, the subarray [5, 3, 6] gives the maximum sum of 14. The key to solving this problem is to use the sliding window technique. Initialize a window of size k and calculate its sum. Then, as the window slides through the array, update the sum by subtracting the element that goes out of the window and adding the new element coming into the window. Keep track of the maximum sum encountered during this process.
Question 2: String Matching
You are given a text string text and a pattern string pattern. You need to find all starting indices in the text where the pattern appears as a substring. The pattern may contain special characters. For example, if text = "ababcabcacbab" and pattern = "abc*", the starting indices should be 1 and 4 (since “abc” at index 1 and “abca” at index 4 match the pattern where * can represent zero or more characters). This problem can be solved using a modified version of the string matching algorithm. One approach could be to iterate through the ¨C19C string, and for each position, try to match the ¨C20C character by character, handling the special characters according to their defined rules.
Question 3: Graph Connectivity Problem
You are given a graph represented as an adjacency list. Each node in the graph is labeled with an integer from 1 to n. The adjacency list graph is a list of lists, where graph[i] contains all the nodes that node i is connected to. You need to determine the number of connected components in the graph.
For example, if graph = [[1, 2], [2, 1], [3, 4], [4, 3]], the graph has two connected components: one consisting of nodes 1 and 2, and the other consisting of nodes 3 and 4.
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