样例过了，WA40

P2341

评测记录

#include <bits/stdc++.h>

using namespace std;
const int MAXN = 100002;

int n,m;
int tid;
int cnt;
int cc[MAXN];
int chu[MAXN];
stack <int> s;
int dfn[MAXN];
int low[MAXN];
int scc[MAXN];
bool ins[MAXN];
vector <int> links[MAXN];
vector <int> links2[MAXN];

void Tarjan(int u)
{
    s.push(u);
    ins[u] = true;
    dfn[u] = low[u] = ++tid;

    auto &link = links[u];
    for(auto v : link)
    {
        if(!dfn[v])
        {
            Tarjan(v);
            low[u] = min(low[u],low[v]);
        }
        else if(ins[v])
            low[u] = min(low[u],dfn[v]);
    }

    if(dfn[u] == low[u])
    {
        int top;
        cnt++;
        do
        {
            top = s.top();
            s.pop();
            cc[cnt]++;
            scc[top] = cnt;
            ins[top] = false;
        }
        while(top != u);
    }
}

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

    cin >> n >> m;
    for(int i = 1;i <= n;i++)
    {
        int u,v;
        cin >> u >> v;
        links[u].push_back(v);
    }

    for(int i = 1;i <= n;i++)
        if(!dfn[i])
            Tarjan(i);
    
    for(int u = 1;u <= n;u++)
    {
        int u2 = scc[u];
        auto &link = links[u];
        auto &link2 = links2[u];
        for(auto v : link)
        {
            int v2 = scc[v];
            if(u2 != v2)
            {
                links[u2].push_back(v2);
                chu[u2]++;
            }
        }
    }

    int ss = 0;
    for(int i = 1;i <= cnt;i++)
        if(chu[i] == 0)
            ss++;
    
    if(ss != 1)
        cout << 0 << endl;
    else
    {
        for(int i = 1;i <= cnt;i++)
        {
            if(chu[i] == 0)
            {
                cout << cc[i] << endl;
                return 0;
            }
        }
    }

    return 0;
}