串行七轮并行要几轮

还是那七个构建任务。这次机器不限：一轮里所有「依赖都做完了」的任务可以同时开工。

下面这段把任务分成一层一层，每一层里的任务可以同时开工。输出串行轮数和并行轮数：

#include <algorithm>
#include <climits>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <utility>
#include <vector>
using namespace std;

using Edges = vector<pair<int, int>>;
const int N = 7;
// 七个构建任务，(a, b) 表示 a 做完才能做 b
const Edges DEP = {{0, 1}, {0, 2}, {1, 3}, {2, 3}, {3, 4}, {4, 5}, {4, 6}};
// 同一张图再加一条 5 → 3，于是有了环
const Edges G = {{0, 1}, {0, 2}, {1, 3}, {2, 3}, {3, 4}, {4, 5}, {5, 3}, {4, 6}};

vector<vector<int>> build(const Edges& edges, int n = N) {
    vector<vector<int>> g(n);
    for (auto [a, b] : edges) g[a].push_back(b);
    for (auto& adj : g) sort(adj.begin(), adj.end());
    return g;
}

// 分层：这一层全做完，入度降到 0 的那些点构成下一层
vector<vector<int>> layers(const Edges& edges) {
    vector<vector<int>> g = build(edges);
    vector<int> d(N, 0);
    for (auto [a, b] : edges) d[b]++;
    vector<int> cur;
    for (int u = 0; u < N; u++) {
        if (d[u] == 0) cur.push_back(u);
    }
    vector<vector<int>> out;
    while (!cur.empty()) {
        out.push_back(cur);
        vector<int> nxt;
        for (int u : cur) {
            for (int v : g[u]) {
                d[v]--;
                if (d[v] == 0) nxt.push_back(v);
            }
        }
        sort(nxt.begin(), nxt.end());
        cur = nxt;
    }
    return out;
}

int main() {
    cout << N << "/" << layers(DEP).size() << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）