P1330 封锁阳光大学 · znzryb

## 思路讲解

01染色问题

## AC代码

[https://www.luogu.com.cn/record/210621292](https://www.luogu.com.cn/record/210621292)

```cpp
// Problem: P1330 封锁阳光大学
// Contest: Luogu
// URL: https://www.luogu.com.cn/problem/P1330
// Memory Limit: 125 MB
// Time Limit: 1000 ms
// by znzryb
// 
// Powered by CP Editor (https://cpeditor.org)

#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define ROF(i, a, b) for (int i = (a); i >= (b); --i)
#define all(x) x.begin(),x.end()
#define CLR(i,a) memset(i,a,sizeof(i))
#define fi first
#define se second
#define pb push_back
#define SZ(a) ((int) a.size())

using namespace std;

typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
typedef array<ll,3> arr;
typedef double DB;
typedef pair<DB,DB> pdd;
typedef pair<ll,bool> plb;
constexpr ll MAXN=static_cast<ll>(1e6)+10,INF=static_cast<ll>(5e18)+3;

ll N,M,T,A[MAXN];
vector<ll> g[MAXN];
ll color[MAXN];
ll sum=0;
ll cnt=0;
bool abNormal=false;

void dfs(ll x,ll xcolor){
	if(abNormal) return;
	if(color[x]!=0 && color[x]!=xcolor){
		abNormal=true;
		return;
	}else if(color[x]!=0){
		return;
	}
	color[x]=xcolor;
	sum+=1;
	if(xcolor==2) cnt+=1;
	for(int i=0;i<g[x].size();++i){
		ll v=g[x][i];
		dfs(v,3-xcolor);
	}
}

inline void solve(){
	cin>>N>>M;
	FOR(i,1,M){
		ll u,v;
		cin>>u>>v;
		g[u].pb(v);
		g[v].pb(u);
	}
	ll ans=0;
	FOR(i,1,N){
		if(color[i]!=0) continue;
		sum=0;
		cnt=0;
		abNormal=false;
		dfs(i,1);
		if(abNormal){
			cout<<"Impossible\n";
			return;
		}
		if(cnt>sum-cnt){
			ans+=sum-cnt;
		}else{
			ans+=cnt;
		}
	}
	cout<<ans<<"\n";
}

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);cout.tie(0);
	// cin>>T;
	// while(T--){
		// solve();
	// }
	solve();
	return 0;
}
/*
AC
https://www.luogu.com.cn/record/210621292
*/
```

## 心路历程（WA，TLE，MLE……）