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https://www.luogu.org/problemnew/show/P2341

Tarjan 缩点 + 判断出度

#include <iostream>
#include <cstdio>
#include <algorithm>

using namespace std;
const int N = 1e4 + ;
const int M = 5e4 + ;

#define yxy getchar()

int Tarjan_tim, now = , n, m, topp, bel;
int Stack[N], head[N], dfn[N], low[N], belong[N], Cnt[N], Out[N];
struct Node {int u, v, nxt;} G[M];
bool vis[N];

inline int read(){
    int x = ; char c = yxy;
    while(c < '' || c > '') c = yxy;
    while(c >= '' && c <= '') x = x *  + c - '', c = yxy;
    return x;
}

inline void add(int u, int v){
    G[now].u = u; G[now].v = v; G[now].nxt = head[u]; head[u] = now ++;
}

void Tarjan(int u){
    dfn[u] = low[u] = ++ Tarjan_tim;
    vis[u] = ;
    Stack[++ topp] = u;
    for(int i = head[u]; ~ i; i = G[i].nxt){
        int v = G[i].v;
        if(!dfn[v]) {
            Tarjan(v);
            low[u] = min(low[u], low[v]);
        }
        else if(vis[v]) low[u] = min(low[u], low[v]);
    }
    if(dfn[u] == low[u]){
        vis[u] = ; belong[u] = ++ bel;
        while(Stack[topp] != u){
            vis[Stack[topp]] = ;
            belong[Stack[topp]] = bel;
            topp --;
        }
        topp --;
    }
}

int main()
{
    n = read(); m = read();
    for(int i = ; i <= n; i ++) head[i] = -;
    for(int i = ; i <= m; i ++) {
        int u = read(); int v = read();
        add(u, v);
    }
    for(int i = ; i <= n; i ++) if(!dfn[i]) Tarjan(i);
    for(int u = ; u <= n; u ++){
        for(int i = head[u]; ~ i; i = G[i].nxt){
            int v = G[i].v;
            if(belong[u] != belong[v]) Out[belong[u]] ++;
        }
        Cnt[belong[u]] ++;
    }
    int js = , Ans;
    for(int i = ; i <= bel; i ++){
        if(!Out[i]) {js ++; Ans = Cnt[i];}
    }
    if(js != ) cout << "";
    else cout << Ans;
    return ;
}