Submission #733358

#	Time	Username	Problem	Language	Result	Execution time	Memory
733358		myrcella	The Xana coup (BOI21_xanadu)	C++17	100 / 100	75 ms	21580 KiB

xanadu

//by szh
#include<bits/stdc++.h>
using namespace std;

#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pb push_back
#define debug(x) cerr<<#x<<"="<<x<<endl
#define pq priority_queue
#define inf 0x3f
#define rep(i,a,b) for (int i=a;i<(b);i++)
#define MP make_pair
#define SZ(x) (int(x.size()))
#define ll long long
#define mod 1000000007
#define ALL(x) x.begin(),x.end()
void inc(int &a,int b) {a=(a+b)%mod;}
void dec(int &a,int b) {a=(a-b+mod)%mod;}
int lowbit(int x) {return x&(-x);}
ll p0w(ll base,ll p) {ll ret=1;while(p>0){if (p%2ll==1ll) ret=ret*base%mod;base=base*base%mod;p/=2ll;}return ret;}

const int maxn = 1e5+10;

int n;
vector <int> edge[maxn];
int on[maxn];
int f[maxn][2][2];

void dfs(int u,int fa) {
	int tmp[2][2][2]; //(i,j,k) j:press?, k:on/off
	int cur=0,lst=1;
	tmp[lst][0][0] = tmp[lst][0][1] = 0;
	tmp[lst][1][0] = tmp[lst][1][1] = maxn;
	for (int v:edge[u]) {
		if (v==fa) continue;
		dfs(v,u);
		rep(i,0,2) {
			tmp[cur][0][i] = min(tmp[lst][0][i]+f[v][0][i],tmp[lst][1][i]+f[v][1][i]);
			tmp[cur][1][i] = min(tmp[lst][0][i]+f[v][1][i],tmp[lst][1][i]+f[v][0][i]);
		}
		swap(cur,lst);
	}
	//press u
	f[u][1][on[u]] = tmp[lst][1][1] + 1;
	f[u][1][on[u]^1] = tmp[lst][0][1] + 1;
	//does not press u
	f[u][0][on[u]] = tmp[lst][0][0];
	f[u][0][on[u]^1] = tmp[lst][1][0];
	//rep(i,0,2) rep(j,0,2) cout<<u<<" "<<i<<" "<<j<<" "<<f[u][i][j]<<endl;
	return;
}

int main() {
//	freopen("input.txt","r",stdin);	
	std::ios::sync_with_stdio(false);cin.tie(0);
	cin>>n;
	rep(i,1,n) {
		int u,v;
		cin>>u>>v;
		edge[u].pb(v);
		edge[v].pb(u);
	}
	rep(i,1,n+1) cin>>on[i];
	dfs(1,-1);
	if (min(f[1][0][0],f[1][1][0])>n) cout<<"impossible";
	else cout<<min(f[1][0][0],f[1][1][0]);
	return 0;
}

• Subtask 1: 5 / 5
• Subtask 2: 15 / 15
• Subtask 3: 10 / 10
• Subtask 4: 40 / 40
• Subtask 5: 30 / 30