If we remove the monster at a position i before removing the monster at position i-1 then we won't be able to utilize the explosion of the monster at position i-1. And since, we want to maximize the utilisation of explosions so as to reduce the number of bullets used, we remove the monsters in the order they are standing. In this way, only the explosion of the monster removed last will be wasted.

Since S[i] is storing numbers, won't it take O(logN) time for each add/delete operation and that too when we use some container like set. Otherwise delete operation could take O(N) time. Please correct me if I'm wrong.
Although we can solve this question if instead of storing numbers that appear exactly i times, we store the count of such numbers in S[i], I found this technique really useful. Edit: Thought we just need to output the mode number's count. Thanks!