I think so.

Putting the code in compiler explorer, you can see the assembly generated when using count() as boolean has one less std::_Rb_tree_increment(std::_Rb_tree_node_base const*) loop.

Meanwhile the modified one that uses count(a) == 10 has

        call    std::_Rb_tree_increment(std::_Rb_tree_node_base const*)
        add     rbp, 1
        mov     rdi, rax
        cmp     r12, rax
        jne     .L71
        cmp     r13, r14
        mov     rax, r14
        cmovge  rax, r13
        cmp     rbp, 10
        cmove   r13, rax
        jmp     .L75

which seems to be the process of count() as it has a cmp rbp, 10 (comparing if the count == 10).

lucky_clover_ Bedge

It was really, and I did a test.

#include <bits/stdc++.h>
using namespace std;

int main() {
    multiset<int> s;
    int x, y;
    cin >> x >> y;
    // Do not let the compiler know the values of x and y in advance, and you should input them equal to 1.
    // This method may be a bit foolish
    for (int i = 1; i <= 100000; ++i) s.insert(min(rand() % x + 1, x));
    // Confusing the compiler, in fact, this always equals to 1.
    for (int i = 1; i <= 100000; ++i) {
        if (s.count(y));
        else return -1; // Never return -1，because x = y = 1.
        y = max(rand() % y + 1, y); //The same
    }
    return 0;
}

When entering 1 1, the result of not turning on O2 in this code is stuck, while when turned on, it ends quickly.

Similarly, I think O2 has turned if (s.count(y)) into if (s.find(y) != s.end()). This greatly improves the running speed of the program. Oh, I didn't know O2 was so powerful before.

Finally, I am a middle school student. And if there are any errors, please feel free to point them out.

When I checked this myself, I noticed a very unusual behavior. I tried submitting the following two programs:

Code-1:

#include <bits/stdc++.h>
using namespace std;
int main()
{
    multiset<int> s;
    for (int i = 1; i <= 100000; ++i)
        s.insert(1);
    int x = 0;
    for (int i = 1; i <= 100000; ++i)
         x = s.count(1);
    cout<<x<<'\n';
    return 0;
}

Code-2:

#include <bits/stdc++.h>
using namespace std;

int main()
{
    multiset<int> s;
    for (int i = 1; i <= 100000; ++i)
        s.insert(1);
    int x = 0;
    for (int i = 1; i <= 100000; ++i)
         x = s.count(1);
    // cout<<x<<'\n';
    return 0;
}

The only difference between 2 codes is I commented cout<<x<<'\n; line. I checked both the programs on Codeforces and Codechef compilers.

Runtime Comparison:

Code-1: Runtime = 15000+ ms (causing TLE)

Code-2: Runtime = 62 ms

I seek to understand the reasons for this drastic increase in time complexity and runtime caused by this seemingly minor modification.

This optimization is inconsistent. This solution TLEs with C++20 but passes with C++23. Replacing count with contains lets it pass with C++20. I haven't been able to replicate this with a simpler program in custom invocation so I don't know why it works in C++23 but not C++20.