POJ2186 Popular Cows

/*
ID: LinKArftc
PROG: 2186.cpp
LANG: C++
*/

#include <map>
#include <set>
#include <cmath>
#include <stack>
#include <queue>
#include <vector>
#include <cstdio>
#include <string>
#include <utility>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;
#define eps 1e-8
#define randin srand((unsigned int)time(NULL))
#define input freopen("input.txt","r",stdin)
#define debug(s) cout << "s = " << s << endl;
#define outstars cout << "*************" << endl;
const double PI = acos(-1.0);
const int inf = 0x3f3f3f3f;
const int INF = 0x7fffffff;
typedef long long ll;

const int maxn = 10010;
const int maxm = 50010;
int n, m;

struct Edge {
    int v, next;
} edge[maxm];

int head[maxn], tot;

void init() {
    tot = 0;
    memset(head, -1, sizeof(head));
}

void addedge(int u, int v) {
    edge[tot].v = v;
    edge[tot].next = head[u];
    head[u] = tot ++;
}

int dfn[maxn], low[maxn], belong[maxn];//每个点属于哪个连通分量
int scc;//连通分量总数,下标 1~scc
int Time;
int ins[maxn];
stack <int> s;
vector <int> vec[maxn];//记录每个连通分量内的点

void tarjan(int u) {
    int v;
    low[u] = dfn[u] = ++ Time;
    s.push(u);
    ins[u] = true;
    for (int i = head[u]; i + 1; i = edge[i].next) {
        v = edge[i].v;
        if (!dfn[v]) {
            tarjan(v);
            low[u] = min(low[u], low[v]);
        } else if (ins[v]) low[u] = min(low[u], dfn[v]);//写出low[u] = min(low[u], low[v]);也可以
    }
    if (low[u] == dfn[u]) {
        scc ++;
        do {
            v = s.top();
            s.pop();
            ins[v] = false;
            belong[v] = scc;
            vec[scc].push_back(v);
        } while (u != v);
    }
}

int outdeg[maxn];

int main() {
    //input;
    while (~scanf("%d %d", &n, &m)) {
        init();
        int u, v;
        for (int i = 1; i <= m; i ++) {
            scanf("%d %d", &u, &v);
            addedge(u, v);
        }
        while (!s.empty()) s.pop();
        for (int i = 1; i <= maxn; i ++) vec[i].clear();
        memset(dfn, 0, sizeof(dfn));
        memset(ins, 0, sizeof(ins));
        Time = 0; scc = 0;
        for (int i = 1; i <= n; i ++) {
            if (!dfn[i]) tarjan(i);
        }
        memset(outdeg, 0, sizeof(outdeg));
        for (int u = 1; u <= n; u ++) {
            for (int i = head[u]; i + 1; i = edge[i].next) {
                int v = edge[i].v;
                if (belong[u] == belong[v]) continue;
                outdeg[belong[u]] ++;
            }
        }
        int ans = 0;
        bool has = false;
        bool flag = false;
        int ii;
        for (int i = 1; i <= scc; i ++) {
            if (outdeg[i] == 0) {
                ii = i;
                if (has) {
                    flag = false;
                    break;
                } else {
                    has = true;
                    flag = true;
                }
            }
        }
        if (flag) printf("%d\n", vec[ii].size());
        else printf("0\n");
    }
    return 0;
}