题解 | #食物链计数#

1. 将食物链按照拓扑序排序
2. 直接即可

#include <bits/stdc++.h>

#define x first
#define y second
#define all(x) x.begin(), x.end()

using namespace std;
using i128 = __int128;
using u128 = unsigned __int128;
using LL = long long;
using LD = long double;
using ULL = unsigned long long;
using PII = pair<int, int>;
using PLL = pair<LL, LL>;
using PLD = pair<LD, LD>;

const int N = 1e5 + 10, MOD = 998244353;
const int INF = 1e9;
const LL LL_INF = 1e18;
const LD EPS = 1e-8;
const int dx4[] = {-1, 0, 1, 0}, dy4[] = {0, 1, 0, -1};
const int dx8[] = {-1, -1, -1, 0, 0, 1, 1, 1}, dy8[] = {-1, 0, 1, -1, 1, -1, 0, 1};

istream &operator>>(istream &is, i128 &val) {
    string str;
    is >> str;
    val = 0;
    bool flag = false;
    if (str[0] == '-') flag = true, str = str.substr(1);
    for (char &c: str) val = val * 10 + c - '0';
    if (flag) val = -val;
    return is;
}

ostream &operator<<(ostream &os, i128 val) {
    if (val < 0) os << "-", val = -val;
    if (val > 9) os << val / 10;
    os << static_cast<char>(val % 10 + '0');
    return os;
}

struct Hash {
    vector<int> h, p;
    int B = 131;
    Hash (const string &s) {
        int n = s.size();
        h.resize(n + 1, 0);
        p.resize(n + 1, 1);
        for (int i = 0; i < n; ++i) {
            p[i + 1] = p[i] * B % MOD;
            h[i + 1] = (h[i] * B + s[i]) % MOD;
        }
    }

    LL get_hash(int l, int r) {
        LL v = h[r] - h[l - 1] * p[r - l + 1] % MOD;
        v = (v % MOD + MOD) % MOD;
        return v;
    }
};

bool cmp(LD a, LD b) {
    if (fabs(a - b) < EPS) return 1;
    return 0;
}

void solve() {
    int n, m;
    cin >> n >> m;
    vector<vector<int>> g(n + 1);
    vector<int> ideg(n + 1), odeg(n + 1);
    for (int i = 0; i < m; ++i) {
        int a, b;
        cin >> a >> b;
        g[a].push_back(b);
        odeg[a]++, ideg[b]++;
    }

    queue<int> q;
    for (int i = 1; i <= n; ++i) {
        if (ideg[i] == 0) {
            q.push(i);
        }
    }

    vector<int> deg(n + 1);
    vector<int> vec;
    deg = ideg;
    while (q.size()) {
        int u = q.front();
        vec.push_back(u);
        q.pop();
        for (int v : g[u]) {
            deg[v]--;
            if (deg[v] == 0) q.push(v);
        }
    }

    vector<LL> f(n + 1);
    for (int i = 1; i <= n; ++i) {
        if (ideg[i] == 0) f[i] = 1;
    }

    for (int u : vec) {
        for (int v : g[u]) {
            f[v] += f[u];
        }
    }

    LL ans = 0;
    for (int i = 1; i <= n; ++i) {
        if (odeg[i] == 0 && ideg[i] > 0) {
            ans += f[i];
        }
    }

    cout << ans << '\n';
}


int main() {
    ios::sync_with_stdio(false);
    cin.tie(0), cout.tie(0);

    int T = 1;
    while (T--) solve();
    cout << fixed << setprecision(15);

    return 0;
}