四个算法，四个 lambda

（本条路线统一用 g++ -std=c++17 -O0 -Wall 编译）

#include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>

int main() {
    std::vector<int> v{5, 3, 9, 1, 7};

    /* 从大到小排：比较规则写成一个 lambda */
    std::sort(v.begin(), v.end(), [](int a, int b) { return a > b; });

    int big = static_cast<int>(
        std::count_if(v.begin(), v.end(), [](int x) { return x > 4; }));

    int sum = std::accumulate(v.begin(), v.end(), 0);

    std::vector<int> out(v.size());
    std::transform(v.begin(), v.end(), out.begin(),
                   [](int x) { return x * 2; });

    std::cout << v.front() << "/" << big << "/" << sum << "/" << out.front() << "\n";
    return 0;
}