I am doing a problem which requires similar operation in which we have to find element smaller than x then delete it from multiset then find for another x. I am using an approach that is giving TLE for 10^5 constraints

Problem Link My Code

Is my method to find that element wrong or is it anything else. PS -> I am not very clear on how to find Elements Lesser than X in set/multiset and its working I used auto it = *lower_bound(st.rbegin(),st.rend(),a[i],greater()); As I saw it in blog can anyone also explain how it works