Popular Cows POJ - 2186(强连通分量)

#include <iostream>
#include <cstdio>
#include <sstream>
#include <cstring>
#include <map>
#include <cctype>
#include <set>
#include <vector>
#include <stack>
#include <queue>
#include <algorithm>
#include <cmath>
#include <bitset>
#define rap(i, a, n) for(int i=a; i<=n; i++)
#define rep(i, a, n) for(int i=a; i<n; i++)
#define lap(i, a, n) for(int i=n; i>=a; i--)
#define lep(i, a, n) for(int i=n; i>a; i--)
#define rd(a) scanf("%d", &a)
#define rlld(a) scanf("%lld", &a)
#define rc(a) scanf("%c", &a)
#define rs(a) scanf("%s", a)
#define pd(a) printf("%d\n", a);
#define plld(a) printf("%lld\n", a);
#define pc(a) printf("%c\n", a);
#define ps(a) printf("%s\n", a);
#define MOD 2018
#define LL long long
#define ULL unsigned long long
#define Pair pair<int, int>
#define mem(a, b) memset(a, b, sizeof(a))
#define _  ios_base::sync_with_stdio(0),cin.tie(0)
//freopen("1.txt", "r", stdin);
using namespace std;
const int maxn = 1e5 + 10, INF = 0x7fffffff, LL_INF = 0x7fffffffffffffff;
int n, m;
vector<int> G[maxn];
int pre[maxn], lowlink[maxn], sccno[maxn], dfs_clock, scc_cnt, out[maxn];
stack<int> S;
int vis[maxn];

void dfs(int u)
{
    pre[u] = lowlink[u] = ++dfs_clock;
    S.push(u);
    for(int i=0; i<G[u].size(); i++)
    {
        int v = G[u][i];
        if(!pre[v])
        {
            dfs(v);
            lowlink[u] = min(lowlink[u], lowlink[v]);
        }
        else if(!sccno[v])
            lowlink[u] = min(pre[v], lowlink[u]);
    }
    if(lowlink[u] == pre[u])
    {
        scc_cnt++;
        for(;;)
        {
            int x = S.top(); S.pop();
            sccno[x] = scc_cnt;
            if(x == u) break;
        }
    }
}

int main()
{
    int u, v;
    rd(n), rd(m);
    for(int i=0; i<m; i++)
    {
        rd(u), rd(v);
        G[u].push_back(v);
    }
    for(int i=1; i<=n; i++)
        if(!pre[i]) dfs(i);
 //   cout << 111 << endl;
    if(scc_cnt == 1)
    {
        pd(n);
        return 0;
    }
    for(int i=1; i<=n; i++)
        for(int j=0; j<G[i].size(); j++)
        {
            int v = G[i][j];
            if(sccno[i] != sccno[v])
                out[sccno[i]]++;
        }
    int res = 0;
  //  cout << scc_cnt << endl;
//    for(int i=1; i<=scc_cnt; i++)
//        cout << in[i] << endl;

    int cnt = 0;
    for(int i=1; i<=scc_cnt; i++)
        if(out[i] == 0)
            cnt++;
    if(cnt != 1)
        return puts("0"), 0;
    for(int i=1; i<=n; i++)
        if(out[sccno[i]] == 0) res++;

    pd(res);

    return 0;
}