【poj2942】 Knights of the Round Table

// bzoj1013
#include<algorithm>
#include<iostream>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<vector>
#include<cmath>
#define LL long long
#define inf 2147483640
#define eps 1e-7
#define Pi acos(-1.0)
#define free(a) freopen(a".in","r",stdin),freopen(a".out","w",stdout);
using namespace std;
inline int gi() {
	int x=0,f=1;char ch=getchar();
	while (ch<‘0‘ || ch>‘9‘) {if (ch==‘-‘) f=-1;ch=getchar();}
	while (ch>=‘0‘ && ch<=‘9‘) {x=x*10+ch-‘0‘;ch=getchar();}
	return x*f;
}

const int maxn=1010,maxm=1000010;
struct edge {int to,next;}e[maxm<<1];
struct data {int u,v;}s[maxn];
int vis[maxn],dfn[maxn],low[maxn],f[maxn][maxn],pos[maxn];
int ind,cnt,top,n,m,num,p,id[maxn],head[maxn];
vector<int> v[maxn];

void link(int u,int v) {
	e[++cnt].to=v;e[cnt].next=head[u];head[u]=cnt;
}
void Init() {
	for (int i=1;i<=num;i++) v[i].clear();
	cnt=ind=top=num=0;
	memset(head,0,sizeof(head));
	memset(vis,0,sizeof(vis));
	memset(dfn,0,sizeof(dfn));
	memset(low,0,sizeof(low));
	memset(pos,0,sizeof(pos));
	memset(id,0,sizeof(id));
	memset(f,0,sizeof(f));
}
void Tarjan(int x,int fa) {
	dfn[x]=low[x]=++ind;
	for (int i=head[x];i;i=e[i].next) if (e[i].to!=fa) {
			if (!dfn[e[i].to]) {
				s[++top]=(data){x,e[i].to};
				Tarjan(e[i].to,x);
				low[x]=min(low[x],low[e[i].to]);
				if (low[e[i].to]>=dfn[x]) {
					num++;
					while (1) {
						v[num].push_back(s[top].u);v[num].push_back(s[top].v);
						if (s[top].u==x && s[top].v==e[i].to) break;
						top--;
					}
					top--;
				}
			}
			else low[x]=min(low[x],dfn[e[i].to]);
		}
}
bool dfs(int x,int col) {
	id[x]=col;
	for (int i=head[x];i;i=e[i].next)
		if (pos[e[i].to]==p) {
			if (!id[e[i].to]) {
				if (!dfs(e[i].to,col^1)) return 0;
			}
			else if (id[e[i].to]==id[x]) return 0;
		}
	return 1;
}
void work(int x) {
	vis[x]=1;
	for (int i=head[x];i;i=e[i].next)
		if (!vis[e[i].to] && pos[e[i].to]==p) work(e[i].to);
}
int main() {
	while (1) {
		n=gi(),m=gi();
		if (!n && !m) break;
		Init();
		for (int x,y,i=1;i<=m;i++) {
			x=gi(),y=gi();
			f[x][y]=f[y][x]=1;
		}
		for (int i=1;i<=n;i++)
			for (int j=1;j<=n;j++) if (i!=j && !f[i][j]) link(i,j);
		for (int i=1;i<=n;i++) if (!dfn[i]) Tarjan(i,0);
		for (p=1;p<=num;p++) {
			for (int j=0;j<v[p].size();j++) pos[v[p][j]]=p;
			if (!dfs(v[p][0],p*2)) work(v[p][0]);
		}
		int ans=0;
		for (int i=1;i<=n;i++) if (!vis[i]) ans++;
		printf("%d\n",ans);
	}
    return 0;
}