Submission #284290

#	Time	Username	Problem	Language	Result	Execution time	Memory
284290		BeanZ	Mousetrap (CEOI17_mousetrap)	C++14	45 / 100	1140 ms	81400 KiB

mousetrap

#include <bits/stdc++.h>

using namespace std;

#define ll int
#define endl '\n'
const int N = 3e6 + 5;
const int mod = 998244353;
ll n, t, m, ban = 0;
vector<ll> node[1000006];
ll dp[1000006], par[1000006], f[1000006];
void dfsFirst(ll u, ll v){
        par[u] = v;
        for (auto j : node[u]){
                if (j == v) continue;
                dfsFirst(j, u);
                f[u] |= f[j];
        }
        if (u == m) f[u] = 1;
}
void dfs(ll u, ll p){
        dp[u] = 0;
        vector<ll> val;
        for (auto j : node[u]){
                if (j == p || j == ban) continue;
                dfs(j, u);
                val.push_back(dp[j] + 1);
        }
        if (val.size()){
                sort(val.begin(), val.end());
                dp[u]++;
                if (val.size() > 1) dp[u] += val[val.size() - 2];
                dp[u] += max(0, (ll)val.size() - 2);
        }
}
int main(){
        ios_base::sync_with_stdio(false);
        cin.tie(0);
        if (fopen("MOUSETRAP.INP", "r")){
                freopen("MOUSETRAP.INP", "r", stdin);
                freopen("MOUSETRAP.OUT", "w", stdout);
        }
        cin >> n >> t >> m;
        for (int i = 1; i < n; i++){
                ll u, v;
                cin >> u >> v;
                node[u].push_back(v);
                node[v].push_back(u);
        }
        dfsFirst(t, t);
        ll ans = 0;
        ll cnt = 0;
        while (true){
                if (m == t) break;
                ll preans = 0;
                dfs(m, par[m]);
                ll cur = par[m];
                preans = dp[m];
                while (cur != t){
                        preans = preans + node[cur].size() - 2;
                        cur = par[cur];
                }
                //cout << preans << endl;
                ans = max(ans, preans - (cnt > 0));
                cnt++;
                ban = m;
                m = par[m];
        }
        cout << ans;
}
/*
13 13 1
1 2
2 3
2 4
1 5
5 6
5 7
1 8
8 9
8 10
1 11
11 12
11 13

10 1 4
1 2
2 3
2 4
3 9
3 5
4 7
4 6
6 8
7 10

14 4 1
1 2
2 5
2 6
5 7
5 8
8 10
6 9
1 3
3 14
3 11
11 12
11 13
1 4

12 12 1
1 2
1 3
3 4
4 5
4 6
3 7
7 8
7 9
3 10
10 11
10 12
*/

Compilation message (stderr)

mousetrap.cpp: In function 'int main()':
mousetrap.cpp:40:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
   40 |                 freopen("MOUSETRAP.INP", "r", stdin);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
mousetrap.cpp:41:24: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)', declared with attribute warn_unused_result [-Wunused-result]
   41 |                 freopen("MOUSETRAP.OUT", "w", stdout);
      |                 ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 20 / 20
• Subtask 2: 25 / 25
• Subtask 3: 0 / 20
• Subtask 4: 0 / 35