https://codeforces.com/problemset/problem/659/E

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思路:

构造发现如果没有环，让一个点牺牲最大，剩下全部能到；如果有环就没有牺牲

#include<iostream>
#include<vector>
#include<queue>
#include<cstring>
#include<cmath>
#include<map>
#include<set>
#include<cstdio>
#include<algorithm>
#define debug(a) cout<<#a<<"="<<a<<endl;
using namespace std;
const int maxn=1e5+1000;
typedef long long LL;
inline 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-48;ch=getchar();}
return x*f;}
vector<LL>g[maxn];
bool flag=1;
LL tot=0,dfn[maxn];
void dfs(LL u,LL fa){
    dfn[u]=++tot;
    for(LL i=0;i<g[u].size();i++){
        LL v=g[u][i];
        if(v==fa) continue;
        if(!dfn[v]){///后序遍历
            dfs(v,u);
        }
        else if(dfn[v]){
            flag=0;
        }
    }
}
int main(void)
{
  cin.tie(0);std::ios::sync_with_stdio(false);
  LL n,m;cin>>n>>m;
  for(LL i=1;i<=m;i++){
    LL u,v;cin>>u>>v;
    g[u].push_back(v);g[v].push_back(u);
  }
  LL ans=0;
  for(LL i=1;i<=n;i++){
    if(!dfn[i]){
        flag=1;
        dfs(i,-1);
        if(1==flag) ans++;
    }
  }
  cout<<ans<<"\n";
return 0;
}