Counting Robot Container Stacking Operations in C++

I’m trying to solve a problem where a robot stacks containers in a warehouse, and I want to calculate the number of operations it performs. The rules are:

There are N containers arranged in a single row.

Each container has a size Ai (1 ≤ Ai ≤ M).

The robot can pick a container and place it inside a larger container to its right.

A container that already contains another cannot be used again.

I need to count the total number of robot operations (nestings).

Input:

First line: two integers M (max container size) and N (number of containers).

Second line: N integers A1, A2, ..., AN representing the container sizes in order.

Output:

One integer: the number of robot operations performed.

Example Input:

5 10 2 2 1 4 3 2 5 4 2 3

Example Output:

4

Explanation:

The robot nests containers into the first available larger container to its right.

After performing 4 nestings, no more containers can be nested.

What I’ve Tried:

I tried using a vector with upper_bound and lower_bound in reverse order:

for(auto it = arr.rbegin(); it != arr.rend();it++){ int val = *it; auto idx = std::upper_bound(S.begin(), S.end(), val); if (idx != S.end()) { ops++; S.erase(idx); } else { S.insert(std::lower_bound(S.begin(), S.end(), val), val); } }

It works for small examples, but I’m not sure if it’s efficient enough for large N. I also read that using a multiset may be better. I'm currently getting 5/100 with this solution.

Question:

Is this approach correct and efficient?

What is the best way to implement this in C++ to handle N up to 20,000 and M up to 1,000?