제출 #593869

#	제출 시각	아이디	문제	언어	결과	실행 시간	메모리
593869		alextodoran	Mousetrap (CEOI17_mousetrap)	C++17	100 / 100	699 ms	178320 KiB

mousetrap

/**
 ____ ____ ____ ____ ____
||a |||t |||o |||d |||o ||
||__|||__|||__|||__|||__||
|/__\|/__\|/__\|/__\|/__\|

**/

#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const int N_MAX = (int) 1e6;

int N, T, M;
vector <int> adj[N_MAX + 2];

int parent[N_MAX + 2];
int up[N_MAX + 2];
int down[N_MAX + 2];

void dfs (int u) {
    int sons = (int) adj[u].size() - (u != T);
    int max1 = 0, max2 = 0;
    for (int v : adj[u]) {
        if (v != parent[u]) {
            parent[v] = u;
            up[v] = up[u] + (u == T ? 1 : sons);
            dfs(v);
            if (down[v] >= max1) {
                max2 = max1;
                max1 = down[v];
            } else if (down[v] > max2) {
                max2 = down[v];
            }
        }
    }
    if (sons == 0) {
        down[u] = up[u];
    } else if (sons == 1) {
        down[u] = up[u] + 1;
    } else {
        down[u] = max2;
    }
}

int path[N_MAX + 2];
int len;

bool check (int answer) {
    int block = 0;
    for (int i = 1; i <= len; i++) {
        int here = 0;
        int u = path[i];
        for (int v : adj[u]) {
            if (v != parent[u] && v != path[i - 1]) {
                if (block + down[v] > answer) {
                    here++;
                }
            }
        }
        block += here;
        if (block > i) {
            return false;
        }
    }
    return (block <= answer);
}

int main () {
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);

    cin >> N >> T >> M;
    for (int i = 1; i <= N - 1; i++) {
        int u, v;
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    dfs(T);

    int u = M;
    while (u != T) {
        path[++len] = u;
        u = parent[u];
    }
    path[++len] = T;

    for (int i = 1; i <= len; i++) {
        int u = path[i];
        for (int v : adj[u]) {
            if (v != parent[u] && v != path[i - 1]) {
                down[v] -= (len - i) + (i > 1);
            }
        }
    }

    int l = 0, r = N;
    while (l < r) {
        int mid = (l + r) / 2;
        if (check(mid) == true) {
            r = mid;
        } else {
            l = mid + 1;
        }
    }
    cout << l << "\n";

    return 0;
}

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