[USACO5.3]校园网Network of Schools

题目传送门

$Tarjan$缩点，不多说。

#include <bits/stdc++.h>

using namespace std;

#define re register
#define rep(i, a, b) for (re int i = a; i <= b; ++i)
#define repd(i, a, b) for (re int i = a; i >= b; --i)
#define maxx(a, b) a = max(a, b);
#define minn(a, b) a = min(a, b);
#define LL long long
#define inf (1 << 30)

inline int read() {
    int w = 0, f = 1; char c = getchar();
    while (!isdigit(c)) f = c == '-' ? -1 : f, c = getchar();
    while (isdigit(c)) w = (w << 3) + (w << 1) + (c ^ '0'), c = getchar();
    return w * f;
}

const int maxn = 100 + 5;

vector<int> G[maxn];
int pre[maxn], low[maxn], dfs_clock = 0, sccno[maxn], sc = 0;
stack<int> s;

void dfs(int u) {
    pre[u] = low[u] = ++dfs_clock;
    s.push(u);
    for (register int i = 0; i < G[u].size(); ++i) {
        register int v = G[u][i];
        if (!pre[v]) {
            dfs(v);
            minn(low[u], low[v]);
        } else
        if (!sccno[v]) minn(low[u], pre[v]);
    }
    if (pre[u] == low[u]) {
        sc++; int x = 0;
        while (x != u) {
            x = s.top(); s.pop();
            sccno[x] = sc;
        }
    }
}

int n, b[maxn];

int main() {
    n = read();

    rep(i, 1, n) {
        int x;
        while (x = read()) G[i].push_back(x);
    }

    rep(i, 1, n)
        if (!sccno[i]) dfs(i);

    int ans = 0;
    if (sc != 1) {
        rep(i, 1, n)
            for (register int j = 0; j < G[i].size(); ++j)
                if (sccno[i] != sccno[G[i][j]]) b[sccno[i]] = 1;
        rep(i, 1, sc) if (!b[i]) ans++;
        memset(b, 0, sizeof(b));
        int ans1 = 0;
        rep(i, 1, n)
            for (register int j = 0; j < G[i].size(); ++j)
                if (sccno[i] != sccno[G[i][j]]) b[sccno[G[i][j]]] = 1;
        rep(i, 1, sc) if (!b[i]) ans1++;
        printf("%d
", ans1);
        maxx(ans, ans1);
    } else printf("1
");
    printf("%d", ans);

    return 0;
}