tarjan+缩点

3 3
1 2
2 1
2 3

1

在对图进行建立时，如果用矩阵会导致内存超限，用VEC会使后来的缩点变得麻烦，所以用邻接表是最佳选择：

#include<stdio.h>
#include<stack>;
#include<string.h>
using namespace std;
stack<int>S;
int dfn[10005],map[10005][110],low[10005],instack[10005],belong[10005],out[10005];
int index,bnt;
void tarjan(int i){
	dfn[i] = low[i] = ++index;
	S.push(i);
	instack[i] = 1;
	for(int j=1;j<=map[i][0];j++){
		int k = map[i][j];
		if(!dfn[k]){
			tarjan(k);
			low[i] = min(low[i],low[k]);
		}
		else if(instack[k]){
			low[i] = min(low[i],dfn[k]);
		}
		
	}
	if(low[i]  == dfn[i]){
		bnt ++;
		int k;
		do{
		    k = S.top();
			S.pop();
			instack[k] = 0;
			belong[k] = bnt;
		}while(i!=k);
	}
}
int main(){
	int m,n,sum;
	while(~scanf("%d%d",&n,&m)){
		int a,b,j;
	   index = bnt = sum = 0;
		memset(map,0,sizeof(map));
		memset(instack,0,sizeof(instack));
		memset(out,0,sizeof(out));
		memset(dfn,0,sizeof(dfn));
		for(int i=0;i<m;i++){
			scanf("%d%d",&a,&b);
			map[a][++map[a][0]] = b;
		}
		for(int i=1;i<=n;i++)
		if(!dfn[i]){
			tarjan(i);
		}
		for(int i=1;i<=n;i++){
			for(j = 1;j<=map[i][0];j++)
			if(belong[i]!=belong[map[i][j]])
			out[belong[i]]++;
		}
		int z=0;
		for(int i=1;i<=bnt;i++){
			if(out[i]==0){
				z++;
				for(int j=1;j<=n;j++)
				if(belong[j]==i)sum++;
			}
		}
		if(z==1)
		printf("%d\n",sum);
		else printf("0\n");
	
	}
}