Tarjan算法模板(USACO03FALL受欢迎的牛)

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#include<bits/stdc++.h>
using namespace std;

const int N = 10010, M = 50010;

int n, m;
int h[N], e[M], ne[M], idx;
int dfn[N], low[N], timestamp;
int stk[N], top;
bool in_stk[N];
int id[N], scc_cnt, Size[N];
int dout[N];

void add(int a, int b){
    e[idx] = b, ne[idx] = h[a], h[a] = idx ++ ;
}

void tarjan(int u){
    dfn[u] = low[u] = ++ timestamp;
    stk[ ++ top] = u, in_stk[u] = true;
    for (int i = h[u]; i != -1; i = ne[i]){
        int j = e[i];
        if (!dfn[j]){
            tarjan(j);
            low[u] = min(low[u], low[j]);
        }
        else if (in_stk[j]) low[u] = min(low[u], dfn[j]);
    }

    if (dfn[u] == low[u]){
        ++ scc_cnt;
        int y;
        do{
            y = stk[top -- ];
            in_stk[y] = false;
            id[y] = scc_cnt;
            Size[scc_cnt] ++ ;
        }while (y != u);
    }
}

int main(){
    scanf("%d%d", &n, &m);
    memset(h, -1, sizeof h);
    while (m -- ){
        int a, b;
        scanf("%d%d", &a, &b);
        add(a, b);
    }

    for (int i = 1; i <= n; i ++ )
        if (!dfn[i])
            tarjan(i);

    for (int i = 1; i <= n; i ++ )
        for (int j = h[i]; ~j; j = ne[j]){
            int k = e[j];
            int a = id[i], b = id[k];
            if (a != b) dout[a] ++ ;
        }

    int zeros = 0, sum = 0;
    for (int i = 1; i <= scc_cnt; i ++ )
        if (!dout[i]){
            zeros ++ ;
            sum += Size[i];
            if (zeros > 1){
                sum = 0;
                break;
            }
        }

    printf("%d\n", sum);

    return 0;
}