Visa Software Engineer I Interview Experience — CodeSignal OA and Two Technical Rounds

CodeSignal online assessment

1. Array operations: given arrays A and B, count the pairs for which A[i] + B[j] equals a specified target.
2. Matrix rotation and boundary handling: rotate an N by N matrix clockwise, then perform special calculations on boundary elements according to specified conditions.
3. String processing and sliding window: find substrings in a long string that match a specified pattern and return their starting indices.
4. Dynamic programming or graph theory: in a grid containing obstacles and weighted cells, find the minimum-cost path from the start to the destination. The time and space requirements were extremely strict, and the solution had to be optimized to the best possible time complexity.

Technical Round 1

1. LeetCode 200, Number of Islands.

Requirements:

1. First implement it with DFS and explain the approach and its time and space complexity in detail.

2. Then implement it again with BFS and compare the advantages, disadvantages, and appropriate use cases for the two approaches.

3. Java core concepts and concurrency.

4. Explain HashMap's internal implementation in detail, including the threshold for converting to a red-black tree and how hash collisions are handled.

5. Explain why HashMap is not thread-safe and what problems can occur in a multithreaded environment.

6. Explain how ConcurrentHashMap is implemented and how it achieves thread safety through segmented locking and CAS operations.

Technical Round 2

1. Shortest-path problem.

Implement Dijkstra's algorithm to find the shortest path between two points in a weighted undirected graph. Use an adjacency list to represent the graph and a PriorityQueue for performance, then analyze the algorithm's time and space complexity.

2. Database design and SQL optimization.

3. Design the database schema for a simple e-commerce system, including user, product, and order tables with appropriate foreign-key relationships.

4. Write a SQL query that returns the ten users with the highest spending during the past 30 days.

5. Explain how an index works and how adding an index could optimize the SQL query above.