(set+拓撲排序) CF1572A Book

題意：

思路:
每本書之間很明視訊記憶體在拓撲關係，由此想到拓撲排序。使用set對圖進行拓撲排序，將閱讀次數小的放在前面，若閱讀次數相同則按照閱讀章節編號排序。假設第 x 章在第
y 章理解之後就能理解，若 x 大於 y 則本次閱讀就可以理解 x 章，否則需要下一次才能理解第 x 章。

code:

#include <bits/stdc++.h>
#include<bits/extc++.h>
using namespace __gnu_pbds;
using namespace std;
using i64 = long long;
using u64 = unsigned long long;
using PII = pair<int, int>;
const int inf = 0x3f3f3f3f;
const i64 INF = 0x3f3f3f3f3f3f3f3f;
#define Z cout << "\n"
#define lb lower_bound
#define ub upper_bound
#define D(x) cerr << #x << ": " << (x) << "\n"
#define DV(v) cerr<<#v<<": ";for(int i=0;i<(v).size();i++)cerr<<((v)[i])<<",";cerr<<"\n"
#if 0
#define int i64
#endif
signed main() {
    ios::sync_with_stdio(false), cin.tie(nullptr), cout.tie(nullptr);
    int t;
    cin >> t;
    while (t--) {
        int n;
        cin >> n;
        vector<vector<int>>g(n + 5);
        vector<int>ld(n + 5);
        for (int i = 1; i <= n; i++) {
            int m;
            cin >> m;
            for (int j = 1; j <= m; j++) {
                int x; cin >> x;
                g[x].push_back(i);
                ld[i]++;
            }
        }
        auto topsort = [&] ()->PII {
            int res = 0, tot = 0;
            multiset<PII>se;
            for (int i = 1; i <= n; i++) {
                if (ld[i] == 0)se.insert({ 1,i });
            }
            while (se.size()) {
                PII q = *se.begin();
                se.erase(se.begin());
                tot++;
                res = max(res, q.first);
                for (auto v : g[q.second]) {
                    if (--ld[v] == 0) {
                        if (v > q.second) {
                            se.insert({ q.first,v });
                        }
                        else {
                            se.insert({ q.first + 1,v });
                        }
                    }
                }
            }
            return { res,tot };
        };
        auto [u, v] = topsort();
        if (v < n)cout << -1 << "\n";
        else cout << u << "\n";
    }
    return 0;
}