I recently gave phone screen for MongoDB (Atlas Search Team). I have 8 YOE and this was a senior software engineer role. Here's the interview question (it was a multi fold question): Implement Inverted search. Implement following functions: - insert(String doc) - search(String term) My solution: I asked the interviewer about the size of the doc, which was not a concern according to them. With that info, I decided to break the input down in words and store them in a HashMap like structure where each word would have a set of associated documents. I was able to implement and run that fine. Second part was to implement - delete(String doc) Third part was to implement - andSearch(String term1, String term2) Find docs that contain both given words. Here's my full solution in Java ``` public class InvertedIndex { // Map from term -> set of documents containing that term private Map> map; public InvertedIndex() { map = new HashMap<>(); } // Insert a document into the index public void insert(String doc) { String[] words = doc.toLowerCase().split("\\W+"); for (String word : words) { map.computeIfAbsent(word, k -> new HashSet<>()).add(doc); } } // Search for documents containing a single term public List search(String term) { Set result = map.getOrDefault(term.toLowerCase(), Collections.emptySet()); return new ArrayList<>(result); } // Delete a document from the index public void delete(String doc) { for (Set docs : map.values()) { docs.remove(doc); if (docs.isEmpty()) { map.remove(word); } } } // Search for documents containing BOTH term1 and term2 public List andSearch(String term1, String term2) { Set set1 = map.getOrDefault(term1.toLowerCase(), Collections.emptySet()); Set set2 = map.getOrDefault(term2.toLowerCase(), Collections.emptySet()); Set result = new HashSet<>(set1); result.retainAll(set2); // intersection return new ArrayList<>(result); } } ``` In this solution I had made a couple of mistakes with original implementation - I forgot to delete item from map after there were no docs associated with the key - I couldn't remember what was the function for set intersection in Java (retainAll) Apart from that I had running solution with just minor typos and 1 hint to simplify the code a bit (I was trying to make it more extensible by introducing a class for Document instead of String but I removed it once there was no need for it) I got rejection email the next day and did not get any feedback from the recruiter as well. I am trying to figure out what did I do wrong here. Any help appreciated. PS: If anyone is interviewing for MongoDB for the same team, they might be asked the same question.