Submission #489340

#	Time	Username	Problem	Language	Result	Execution time	Memory
489340		cpp219	The Xana coup (BOI21_xanadu)	C++14	100 / 100	92 ms	13864 KiB

xanadu

#include<bits/stdc++.h>
#define ll int
#define ld long double
#define fs first
#define sc second
#define debug(y) cout<<y,exit(0)
using namespace std;
typedef pair<ll,ll> LL;
const ll N = 1e5 + 9;
const ll inf = 1e9 + 7;

vector<ll> g[N];
ll n,x,y,a[N],dp[N][4];

bool chk(ll x,ll i){
    return (x >> i) & 1;
}

void Combine(ll u,ll v){
    ll tmp[4]; tmp[0] = tmp[1] = tmp[2] = tmp[3] = inf;
    for (ll i = 0;i < 4;i++)
        for (ll j = 0;j < 4;j++){
            if (chk(i,0) && !chk(j,1)) continue;
            if (!chk(i,0) && chk(j,1)) continue;
            ll now = chk(i,1) ^ chk(j,0); now = now*2 + chk(i,0);
            tmp[now] = min(tmp[now],dp[u][i] + dp[v][j]);
        }
    for (ll i = 0;i < 4;i++) dp[u][i] = tmp[i];
}

void DFS(ll u,ll p){
    dp[u][a[u]*2] = 0; dp[u][(1^a[u])*2 + 1] = 1;
    for (auto i : g[u]) if (i != p){
        DFS(i,u); Combine(u,i);
    }
}

int main(){
    ios_base::sync_with_stdio(NULL); cin.tie(0); cout.tie(0);
    #define task "tst"
    if (fopen(task".inp","r")){
        freopen(task".inp","r",stdin);
        //freopen(task".out","w",stdout);
    }
    cin>>n;
    for (ll i = 1;i < n;i++){
        cin>>x>>y;
        g[x].push_back(y); g[y].push_back(x);
    }
    for (ll i = 1;i <= n;i++){
        cin>>a[i];
        for (ll j = 0;j < 4;j++) dp[i][j] = inf;
    }
    DFS(1,0);
    ll kq = min(dp[1][0],dp[1][1]);
    if (kq == inf) cout<<"impossible";
    else cout<<kq;
}

/* stuff you should look for
	* int overflow, array bounds
	* special cases (n=1?)
	* do smth instead of nothing and stay organized
	* WRITE STUFF DOWN
	* DON'T GET STUCK ON ONE APPROACH
*/

Compilation message (stderr)

xanadu.cpp: In function 'int main()':
xanadu.cpp:42:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   42 |         freopen(task".inp","r",stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~

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