Submission #1149137

#	Time	Username	Problem	Language	Result	Execution time	Memory
1149137		KhoaDuy	Mousetrap (CEOI17_mousetrap)	C++20	100 / 100	621 ms	184276 KiB

mousetrap

#include<bits/stdc++.h>
using namespace std;
#define endl '\n'
const int MAXN=1e6;
int DP[MAXN+1];
vector<vector<int>> graph(MAXN+1);
vector<int> path;
bool in[MAXN+1]={false};
bool getpath(int u,int p,int t){
    path.push_back(u);
    if(u==t){
        return true;
    }
    for(int v:graph[u]){
        if(v!=p){
            if(getpath(v,u,t)){
                return true;
            }
        }
    }
    path.pop_back();
    return false;
}
void DFS(int u,int p){
    DP[u]=0;
    vector<int> child;
    for(int v:graph[u]){
        if(v!=p&&!in[v]){
            DFS(v,u);
            child.push_back(DP[v]);
        }
    }
    sort(child.begin(),child.end(),greater<int>());
    DP[u]=child.size();
    if(child.size()>=2){
        DP[u]+=child[1];
    }
}
int cost[MAXN+1];
signed main(){
    //freopen("mousetrap.inp","r",stdin);
    //freopen("mousetrap.out","w",stdout);
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n,t,root;
    cin >> n >> t >> root;
    for(int i=0;i<n-1;i++){
        int u,v;
        cin >> u >> v;
        graph[u].push_back(v);
        graph[v].push_back(u);
    }
    if(root==t){
        cout << 0;
        return 0;
    }
    getpath(root,-1,t);
    for(int i=0;i<path.size();i++){
        in[path[i]]=true;
    }
    int sum=0;
    for(int i=path.size()-2;i>=0;i--){
        int u=path[i],p=-1;
        if(i>0){
            p=path[i-1];
        }
        for(int v:graph[u]){
            if(v!=p&&!in[v]){
                sum++;
            }
        }
        for(int v:graph[u]){
            if(v!=p&&!in[v]){
                DFS(v,-1);
                cost[v]=DP[v]+sum;
            }
        }
    }
    int low=0,high=1e9;
    low--,high++;
    while(high-low>1){
        int mid=((high-low)>>1)+low;
        sum=0;
        bool stop=true;
        for(int i=0;i+1<path.size();i++){
            int u=path[i],p=-1;
            if(i>0){
                p=path[i-1];
            }
            int cnt=0;
            for(int v:graph[u]){
                if(v!=p&&!in[v]&&cost[v]+sum>mid){
                    cnt++;
                }
            }
            sum+=cnt;
            if(sum>i+1){
                stop=false;
                break;
            }
        }
        if(sum>mid){
            stop=false;
        }
        if(!stop){
            low=mid;
        }
        else{
            high=mid;
        }
    }
    cout << high;
}

• Subtask 1: 20 / 20
• Subtask 2: 25 / 25
• Subtask 3: 20 / 20
• Subtask 4: 35 / 35