NOIP2015 提高組 day1 信息傳遞

tarjan求強連通分量大小的裸題

貼代碼

#include<cstdio>
#include<cstring>
#define MAXN 200000
#define INF 1e9
using namespace std;
inline int Min(int a,int b){return a<b?a:b;}
struct node{
	int to,nxt;
}adj[MAXN+5];
int n,ans;
bool ins[MAXN+5];
int sta[MAXN+5],tail;
int head[MAXN+5],ecnt;
int block[MAXN+5],blcnt;
int low[MAXN+5],dfn[MAXN+5],dcnt;
void add(int u,int v)
{
	adj[++ecnt].to=v;
	adj[ecnt].nxt=head[u];
	head[u]=ecnt;
}
void dfs(int u)
{
	int i;
	ins[u]=true;
	sta[++tail]=u;
	low[u]=dfn[u]=++dcnt;
	for(i=head[u];i;i=adj[i].nxt)
	{
		int v=adj[i].to;
		if(dfn[v]==0)
		{
			dfs(v);
			low[u]=Min(low[u],low[v]);
		}
		else if(ins[v])
			low[u]=Min(low[u],dfn[v]);
	}
	if(low[u]==dfn[u])
	{
		++blcnt;
		int sz=0;
		do{
			++sz;
			ins[sta[tail]]=false;
			block[sta[tail--]]=blcnt;
		}while(sta[tail+1]!=u);
		if(sz>1)
			ans=Min(ans,sz);
	}
}
void tarjan()
{
	int i;
	ans=INF;
	memset(dfn,0,sizeof dfn);
	memset(low,0,sizeof low);
	for(i=1;i<=n;i++)
		if(dfn[i]==0)
			dfs(i);
}
int main()
{
	freopen("message.in","r",stdin);
	freopen("message.out","w",stdout);
	int u,v;
	scanf("%d",&n);
	for(u=1;u<=n;u++)
	{
		scanf("%d",&v);
		add(u,v);
	}
	tarjan();
	printf("%d\n",ans);
}