I know 2-D segment tree and 2-D Fenwick tree and I have used them. I was trying to solve the problem 869E - The Untended Antiquity . In this question, I understood the whole core logic of 2-D Fenwick tree but I couldn't understand why we use random instead of that if I use count=0 like assigning count for one boundary then I increment the count and assign that count to another boundary and so on. I tried this approach and I got the wrong answer on Test Case 9 but it was too large that I couldn't see what the test case was. So, if you can tell me why we use random numbers it will be helpful.