Submission #609974

#	Time	Username	Problem	Language	Result	Execution time	Memory
609974		aSSSd	Network (BOI15_net)	C++14	100 / 100	899 ms	138172 KiB

net

#include <bits/stdc++.h>
using namespace std;
#define in ({int x=0;int c=getchar(),n=0;for(;!isdigit(c);c=getchar()) n=(c=='-');for(;isdigit(c);c=getchar()) x=x*10+c-'0';n?-x:x;})
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int rnd(int l,int r)
{
    return l+rng()%(r-l+1);
}
#define fasty ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
#define forinc(x,a,b) for(int x=a;x<=b;x++)
#define fordec(x,a,b) for(int x=a;x>=b;x--)
#define forv(a,b) for(auto&a:b)
#define fi first
#define se second
#define pb push_back
#define ii pair<int,int>
#define mt make_tuple
#define all(a) a.begin(),a.end()
#define reset(f,x) memset(f,x,sizeof(f))
#define getbit(x,i) ((x>>i)&1)
#define batbit(x,i) (x|(1ll<<i))
#define tatbit(x,i) (x&~(1<<i))
#define gg exit(0);
const int N = 5e5 + 100;
vector<int> ke[N];
set<int> adj[N];
int n, m;
int num[N], low[N], it, del[N];
stack<int> st;
int cnt;
int id[N], two[N];
void dfs(int u,int p)
{
	num[u]=low[u]=++it;
	st.push(u);
	for(auto v:ke[u])
	{
	    if(del[v]) continue;
		if(v==p) continue;
		if(!num[v])
		{
			dfs(v,u);
			low[u]=min(low[u],low[v]);
		}
		else  low[u]=min(low[u],num[v]);
	}
	if(num[u]==low[u])
	{
		cnt++;
		int v;
		while(1)
		{
			v=st.top();
			st.pop();
			id[v]=cnt;
			del[v]=1;
			two[cnt] = v;
			if(v==u) break;
		}
	}
}
vector<int> leaf;
void dfs2(int u, int pa)
{
    if(adj[u].size() == 1) leaf.pb(two[u]);

    for(auto v : adj[u])
    {
        if(v == pa) continue;
        dfs2(v,u);
    }
}

main()
{
    fasty;
   // freopen("bluehouse.inp" , "r" ,stdin);
   // freopen("bluehouse.out" , "w", stdout );
  //  freopen("task.inp" , "r", stdin);
    cin >> n;
    m=n-1;
    forinc(i,1,m)
    {
        int u,v;
        cin >> u >> v;
        ke[u].pb(v);
        ke[v].pb(u);
    }
    forinc(i,1,n) if(!num[i]) dfs(i,0);

    forinc(i,1,n)
    {
        for(auto v : ke[i])
        {
            int x = id[i];
            int y = id[v];
            if(x!=y)
            {
                adj[x].insert(y);
                adj[y].insert(x);
            }
        }
    }

    dfs2(1,0);
    int l=0;
    int step = leaf.size()/2;
    if(leaf.size() % 2 == 0)
    {
        cout << leaf.size()/2 << "\n";
        while(l + leaf.size()/2 < leaf.size())
        {
            cout << leaf[l] << " " << leaf[l + step] << "\n";
            l++;
        }
        return 0;
    }
    leaf.pb(leaf[0]);
    step++;
    cout << leaf.size()/2 << "\n";
    while(l + step < leaf.size())
    {
        cout << leaf[l] << " " << leaf[l + step] << "\n";
        l++;
    }
}

Compilation message (stderr)

net.cpp:75:1: warning: ISO C++ forbids declaration of 'main' with no type [-Wreturn-type]
   75 | main()
      | ^~~~
net.cpp: In function 'int main()':
net.cpp:122:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
  122 |     while(l + step < leaf.size())
      |           ~~~~~~~~~^~~~~~~~~~~~~

• Subtask 1: 18 / 18
• Subtask 2: 45 / 45
• Subtask 3: 37 / 37