POJ2186 Popular Cows (强连通缩点)

#include<map>
#include<queue>
#include<cmath>
#include<cstdio>
#include<stack>
#include<iostream>
#include<cstring>
#include<algorithm>
#define LL int
#define inf 0x3f3f3f3f
#define eps 1e-8
#include<vector>
#define ls l,mid,rt<<1
#define rs mid+1,r,rt<<1|1

using namespace std;
const int Ma = 1000005;

int head[Ma],dfn[Ma],num[Ma],du[Ma],stk[Ma],vis[Ma],low[Ma];
int cnt,top,time,css;

struct node{
int to,next;
}q[Ma];

void bu(int a,int b){
q[cnt].to = b;
q[cnt].next = head[a];
head[a] = cnt++;
}

void init(){
time = 1;
css = top = cnt = 0;
memset(dfn,0,sizeof(dfn));
memset(low,0,sizeof(low));
memset(vis,0,sizeof(vis));
memset(head,-1,sizeof(head));
memset(num,0,sizeof(num));
memset(du,0,sizeof(du));
}

void Tarjan(int u){
low[u] = dfn[u] = time++;
vis[u] = 1;
stk[top++] = u;
for(int i = head[u]; ~i ; i = q[i].next){
int v = q[i].to;
if(!dfn[v]){
Tarjan(v);
low[u] = min(low[u],low[v]);
}
else if(vis[v])
low[u] = min(low[u],dfn[v]);
}
if(low[u] == dfn[u]){//找到极大联通分量
css++;
while(top > 0&&stk[top] != u){
top --;
vis[stk[top] ] = 2;
num[stk[top] ] = css;
}
}
}
int main(){
int n,m,i,j,k,a,b;
while(~scanf("%d%d",&n,&m)){
init();
for(i = 0;i < m;++ i){
scanf("%d%d",&a,&b);
bu(a,b);
}
for(i = 1;i <= n;++ i){
if(!dfn[i])
Tarjan(i);
}

for(i = 1;i <= n;++ i){
for(j = head[i]; ~j ; j = q[j].next){
if(num[i]!=num[q[j].to ])
du[num[i] ]++;
}
}

int sum = 0,tmp;
for(i = 1;i <= css;++ i){
if(!du[i]){
sum++;
tmp = i;
}
}
if(sum==1){
sum = 0;
for(i = 1;i <= n;++ i)
if(num[i] == tmp)
sum++;
printf("%d\n",sum);
}
else
printf("0\n");
}
return 0;
}