bzoj 1051 [HAOI2006]受欢迎的牛（tarjan缩点）

题目链接：http://www.lydsy.com:808/JudgeOnline/problem.php?id=1051

题解：缩点之后判断出度为0的有几个，只有一个那么输出那个强连通块的点数，否者就输出0

#include <iostream>
#include <cstring>
#include <cstdio>
#include <map>
using namespace std;
const int N = 1e6 + 10;
const int M = 5e6 + 10;
map<int,int>mmp[N];
struct TnT {
    int v, next;
}edge[M];
int head[N], e;
int Low[N], DFN[N], Stack[N], Belong[N], num[N];
int Index, top, scc;
int Out[N];
bool Instack[N];
void init() {
    memset(head, -1, sizeof(head));
    e = 0;
}
void add(int u, int v) {
    edge[e].v = v;
    edge[e].next = head[u];
    head[u] = e++;
}
void Tarjan(int u) {
    int v;
    Low[u] = DFN[u] = ++Index;
    Stack[top++] = u;
    Instack[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(Instack[v]) Low[u] = min(Low[u] , DFN[v]);
    }
    if(DFN[u] == Low[u]) {
        scc++;
        do {
            v = Stack[--top];
            Instack[v] = false;
            Belong[v] = scc;
            num[scc]++;
        } while(v != u);
    }
}
int main() {
    int n, m;
    scanf("%d%d", &n, &m);
    init();
    for(int i = 0; i < m; i++) {
        int a, b;
        scanf("%d%d", &a, &b);
        if(mmp[a][b] != 1) {
            mmp[a][b] = 1;
            add(a , b);
        }
    }
    memset(DFN, 0, sizeof(DFN));
    memset(Low, 0, sizeof(Low));
    memset(Instack, false, sizeof(Instack));
    memset(num, 0, sizeof(num));
    for(int i = 1; i <= n; i++) {
        if(!DFN[i]) Tarjan(i);
    }
    for(int i = 1; i <= n; i++) {
        for(int j = head[i]; j != -1; j = edge[j].next) {
            int v = edge[j].v;
            if(Belong[i] != Belong[v]) Out[Belong[i]]++;
        }
    }
    int ans = 0 , pos = -1;
    for(int i = 1; i <= scc; i++) {
        if(Out[i] == 0) {ans++, pos = i;}
    }
    if(ans > 1) printf("0\n");
    else printf("%d\n" , num[pos]);
    return 0;
}