洛谷 P2341 [USACO03FALL][HAOI2006]受欢迎的牛 G

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同 AcWing 1174 受欢迎的牛

#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using p = pair<int, int>;
const int maxn(1e4 + 10);
const int maxm(5e4 + 10);
int ecnt, head[maxn];
int tim, dfn[maxn], low[maxn];
int scnt, scc[maxn], siz[maxn], deg[maxn];
bool vis[maxn];
stack<int> st;

struct edge {
    int to, nxt;
} edges[maxm];

template<typename T = int>
inline const T read()
{
    T x = 0, f = 1;
    char ch = getchar();
    while (ch < '0' || ch > '9') {
        if (ch == '-') f = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9') {
        x = (x << 3) + (x << 1) + ch - '0';
        ch = getchar();
    }
    return x * f;
}

template<typename T>
inline void write(T x, bool ln)
{
    if (x < 0) {
        putchar('-');
        x = -x;
    }
    if (x > 9) write(x / 10, false);
    putchar(x % 10 + '0');
    if (ln) putchar(10);
}

void addEdge(int u, int v)
{
    edges[ecnt].to = v;
    edges[ecnt].nxt = head[u];
    head[u] = ecnt++;
}

void tarjan(int u)
{
    dfn[u] = low[u] = ++tim;
    st.push(u);
    vis[u] = true;
    for (int i = head[u]; compl i; i = edges[i].nxt) {
        int v = edges[i].to;
        if (not dfn[v]) {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        } else if (vis[v]) {
            low[u] = min(low[u], dfn[v]);
        }
    }
    if (dfn[u] == low[u]) {
        ++scnt;
        int v = 0;
        do {
            v = st.top();
            st.pop();
            vis[v] = false;
            scc[v] = scnt;
            ++siz[scnt];
        } while (v not_eq u);
    }
}

int main()
{
#ifdef ONLINE_JUDGE
#else
    freopen("input.txt", "r", stdin);
#endif
    ios::sync_with_stdio(false);
    memset(head, -1, sizeof head);
    int n = read(), m = read();
    while (m--) {
        int u = read(), v = read();
        addEdge(u, v);
    }
    for (int i = 1; i <= n; ++i) {
        if (not dfn[i]) {
            tarjan(i);
        }
    }
    for (int u = 1; u <= n; ++u) {
        for (int i = head[u]; compl i; i = edges[i].nxt) {
            int v = edges[i].to;
            int a = scc[u], b = scc[v];
            if (a not_eq b) {
                ++deg[a];
            }
        }
    }
    int cnt = 0, sum = 0;
    for (int i = 1; i <= scnt; ++i) {
        if (not deg[i]) {
            ++cnt;
            sum += siz[i];
            if (cnt > 1) {
                sum = 0;
                break;
            }
        }
    }
    write(sum, true);
    return 0;
}