[BZOJ]4238: 电压

#include <algorithm>
#include <iostream>
#include <cstring>
#include <cstdio>
#include <vector>
#include <stack>
#include <queue>
#include <cmath>
#include <set>
#include <map>
#define mp make_pair
#define pb push_back
#define pii pair<int,int>
#define link(x) for(edge *j=h[x];j;j=j->next)
#define inc(i,l,r) for(int i=l;i<=r;i++)
#define dec(i,r,l) for(int i=r;i>=l;i--)
const int MAXN=3e5+10;
const double eps=1e-8;
#define ll long long
using namespace std;
struct edge{int t;edge*next;}e[MAXN<<1],*h[MAXN],*o=e;
void add(int x,int y){o->t=y;o->next=h[x];h[x]=o++;}
ll read(){
    ll x=0,f=1;char ch=getchar();
    while(!isdigit(ch)){if(ch==‘-‘)f=-1;ch=getchar();}
    while(isdigit(ch))x=x*10+ch-‘0‘,ch=getchar();
    return x*f;
}
int n,m;
typedef struct node{
    int u,v;
}node;
vector<node>vec;
int f[MAXN];
int fa[MAXN],son[MAXN],num[MAXN],dep[MAXN];
int sum[MAXN];

int find1(int x){
    if(x==f[x])return x;
    else return f[x]=find1(f[x]);
}
void dfs(int x,int pre,int deep){
    fa[x]=pre;num[x]=1;dep[x]=deep+1;
    link(x){
	if(j->t==pre)continue;
	dfs(j->t,x,deep+1);
	num[x]+=num[j->t];
	if(son[x]==-1||num[son[x]]<num[j->t])son[x]=j->t;
    }
}
int tp[MAXN];
void dfs1(int x,int td){
    tp[x]=td;
    if(son[x]!=-1)dfs1(son[x],td);
    link(x)if(j->t!=son[x]&&j->t!=fa[x])dfs1(j->t,j->t);
}

int Lca(int x,int y){
    int xx=tp[x];int yy=tp[y];
    while(xx!=yy){
	if(dep[xx]<dep[yy])swap(xx,yy),swap(x,y);
	x=fa[xx];xx=tp[x];
    }
    if(dep[x]>dep[y])swap(x,y);
    return x;
}

void dfs2(int x){
    link(x){
	if(j->t==fa[x])continue;
	dfs2(j->t);
	sum[x]+=sum[j->t];
    }
}

bool vis[MAXN];
int main(){
    n=read(),m=read();
    inc(i,1,n)f[i]=i,son[i]=-1;
    int u,v;
    while(m--){
	u=read();v=read();
	int t1=find1(u);int t2=find1(v);
	if(t1==t2){vec.pb((node){u,v});continue;}
	f[t1]=t2;add(u,v);add(v,u);
    }
    inc(i,1,n)if(!dep[i])dfs(i,0,0),dfs1(i,i),vis[i]=1,sum[i]=-1;
    int cnt=0;
    for(int i=0;i<vec.size();i++){
	u=vec[i].u;v=vec[i].v;
	if(u>v)swap(u,v);
	int lca=Lca(u,v);
	int t=dep[u]+dep[v]-2*dep[lca]+1;
	if(t&1){
	    cnt++,sum[u]++,sum[v]++,sum[lca]-=2;
	}
	else sum[u]--,sum[v]--,sum[lca]+=2;
    }
    int ans=0;
    if(cnt==1)ans++;
    inc(i,1,n)if(vis[i])dfs2(i);
    inc(i,1,n)if(cnt==sum[i])ans++;
    printf("%d\n",ans);
}