//無向圖的關鍵點,dfs鄰接陣形式,O(n^2)
//返回關鍵點個數,key[]返回點集
//傳入圖的大小n和鄰接陣mat,不相鄰點邊權0
#define MAXN 110

void search(int n,int mat[][MAXN],int* dfn,int* low,int now,int& ret,int* key,int& cnt,int root,int& rd,int* bb){
	int i;
	dfn[now]=low[now]=++cnt;
	for (i=0;i<n;i++)
		if (mat[now][i]){
			if (!dfn[i]){
				search(n,mat,dfn,low,i,ret,key,cnt,root,rd,bb);
				if (low[i]<low[now])
					low[now]=low[i];
				if (low[i]>=dfn[now]){
					if (now!=root&&!bb[now])
						key[ret++]=now,bb[now]=1;
					else if(now==root)
						rd++;
				}
			}
			else if (dfn[i]<low[now])
				low[now]=dfn[i];
		}
}

int key_vertex(int n,int mat[][MAXN],int* key){
	int ret=0,i,cnt,rd,dfn[MAXN],low[MAXN],bb[MAXN];
	for (i=0;i<n;dfn[i++]=bb[i]=0);
	for (cnt=i=0;i<n;i++)
		if (!dfn[i]){
			rd=0;
			search(n,mat,dfn,low,i,ret,key,cnt,i,rd,bb);
			if (rd>1&&!bb[i])
				key[ret++]=i,bb[i]=1;
		}
	return ret;
}