装三个、加一遍、拿掉一个

（本条路线统一用 g++ -std=c++17 -O0 -Wall 编译）

#include <iostream>
#include <vector>

int main() {
    std::vector<int> v;
    v.reserve(4);
    v.push_back(10);
    v.push_back(20);
    v.push_back(30);

    int sum = 0;
    for (std::size_t i = 0; i < v.size(); i++) sum += v[i];

    v.pop_back();
    std::cout << v.size() << "/" << sum << "/" << v.back() << "\n";
    return 0;
}