Submission #374476

#	Time	Username	Problem	Language	Result	Execution time	Memory
374476		srvlt	Torrent (COI16_torrent)	C++14	100 / 100	130 ms	32128 KiB

torrent

#include <bits/stdc++.h>
#define ll long long
#define pb push_back
#define mem(x, y) memset(& x, y, sizeof(x))
#define all(x) begin(x), end(x)
#define SZ(x) (int)(x).size()
#define cerr if(dbg)cerr
using namespace std;
bool dbg=1;

const int n0=3e5+123;
int n,a,b,dp[n0],good[n0];
vector<int> g[n0],path;
void dfs(int v,int p) {
	if(v==b) {
		good[v]=1;
	}
	vector<int> ord;
	for(int to:g[v]) {
		if(to==p) continue;
		dfs(to,v);
		good[v]|=good[to];
		if(!good[to]) {
			ord.pb(dp[to]+1);
		}
	}
	sort(all(ord)),reverse(all(ord));
	for(int i=0; i<SZ(ord); i++)
		dp[v]=max(dp[v],ord[i]+i);
	if(good[v]) {
		path.pb(v);
	}
}
int ok[n0];
bool check(int x) {
	mem(ok,0);
	int cur=0;
	for(int i=0; i<SZ(path); i++) {
		if(cur+dp[path[i]]+1<=x) cur++,ok[i]=1;
		else if(cur+dp[path[i]]<=x) cur+=2,ok[i]=1;
		else cur+=2;
	}
	cur=0;
	for(int i=SZ(path)-1; i>=0; i--) {
		if(cur+dp[path[i]]+1<=x) cur++,ok[i]=1;
		else if(cur+dp[path[i]]<=x) cur+=2,ok[i]=1;
		else cur+=2;
		if(!ok[i]) return 0;
	}
	return 1;
}

int main() {
	ios_base::sync_with_stdio(0), cin.tie(0);
	cin >> n >> a >> b;
	for(int i=1; i<n; i++) {
		int x,y;cin >> x >> y;
		g[x].pb(y),g[y].pb(x);
	}
	dfs(a,0);
	assert(SZ(path)>1);
	//for(int i=0; i<SZ(path); i++) {
		//cerr << "path[" << i << "]=" << path[i] << "," << dp[path[i]] << '\n';
	//}
	int l=-1,r=n0;
	while(l<r-1) {
		int mid=l+r>>1;
		if(check(mid)) r=mid;
		else l=mid;
	}
	cout << r;
}

Compilation message (stderr)

torrent.cpp: In function 'int main()':
torrent.cpp:67:12: warning: suggest parentheses around '+' inside '>>' [-Wparentheses]
   67 |   int mid=l+r>>1;
      |           ~^~

• Subtask 1: 31 / 31
• Subtask 2: 69 / 69