Submission #911139

#	Time	Username	Problem	Language	Result	Execution time	Memory
911139		ttamx	Network (BOI15_net)	C++14	100 / 100	828 ms	77140 KiB

net

#include<bits/stdc++.h>

using namespace std;

const int N=5e5+5;

int n;
int sz[N];
vector<int> adj[N];

int dfssz(int u,int p=0){
    sz[u]=(adj[u].size()<2);
    for(auto v:adj[u])if(v!=p)sz[u]+=dfssz(v,u);
    return sz[u];
}

int centroid(int u,int cnt,int p=0){
    for(auto v:adj[u])if(v!=p&&sz[v]>cnt/2)return centroid(v,cnt,u);
    return u;
}

void dfs(int u,int p,vector<int> &comp){
    if(adj[u].size()==1)comp.emplace_back(u);
    for(auto v:adj[u])if(v!=p)dfs(v,u,comp);
}

int main(){
    cin.tie(nullptr)->sync_with_stdio(false);
    cin >> n;
    for(int i=1;i<n;i++){
        int u,v;
        cin >> u >> v;
        adj[u].emplace_back(v);
        adj[v].emplace_back(u);
    }
    int c=1;
    for(int i=1;i<=n;i++)if(adj[i].size()>1)c=i;
    c=centroid(c,dfssz(c));
    vector<vector<int>> res;
    int cnt=0;
    for(auto v:adj[c]){
        res.push_back({});
        dfs(v,c,res.back());
        cnt+=res.back().size();
    }
    int m=res.size();
    priority_queue<pair<int,int>> pq;
    for(int i=0;i<m;i++){
        pq.emplace(res[i].size(),i);
        assert(res[i].size()<=cnt/2);
    }
    vector<pair<int,int>> ans;
    while(!pq.empty()){
        auto [s,i]=pq.top();
        pq.pop();
        if(pq.empty()){
            assert(s==1);
            ans.emplace_back(res[i].back(),c);
            break;
        }
        auto [t,j]=pq.top();
        pq.pop();
        ans.emplace_back(res[i].back(),res[j].back());
        res[i].pop_back();
        res[j].pop_back();
        if(--s)pq.emplace(s,i);
        if(--t)pq.emplace(t,j);
    }
    cout << ans.size() << "\n";
    for(auto [x,y]:ans)cout << x << " " << y << "\n";
}

Compilation message (stderr)

In file included from /usr/include/c++/10/cassert:44,
                 from /usr/include/x86_64-linux-gnu/c++/10/bits/stdc++.h:33,
                 from net.cpp:1:
net.cpp: In function 'int main()':
net.cpp:50:29: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   50 |         assert(res[i].size()<=cnt/2);
      |                ~~~~~~~~~~~~~^~~~~~~
net.cpp:54:14: warning: structured bindings only available with '-std=c++17' or '-std=gnu++17'
   54 |         auto [s,i]=pq.top();
      |              ^
net.cpp:61:14: warning: structured bindings only available with '-std=c++17' or '-std=gnu++17'
   61 |         auto [t,j]=pq.top();
      |              ^
net.cpp:70:14: warning: structured bindings only available with '-std=c++17' or '-std=gnu++17'
   70 |     for(auto [x,y]:ans)cout << x << " " << y << "\n";
      |              ^

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