Submission #1024685

#	Time	Username	Problem	Language	Result	Execution time	Memory
1024685		stdfloat	Torrent (COI16_torrent)	C++17	100 / 100	250 ms	25280 KiB

torrent

#include <bits/stdc++.h>
using namespace std;

using ll = long long;

vector<bool> vis;

vector<int> pr;

vector<vector<int>> E;

void dfs_pr(int x, int p = -1) {
    pr[x] = p;
    for (auto i : E[x]) {
        if (i != p) dfs_pr(i, x);
    }
}

int dfs(int x) {
    vis[x] = true;

    vector<int> v;
    for (auto i : E[x]) {
        if (!vis[i]) v.push_back(dfs(i));
    }

    sort(v.rbegin(), v.rend());

    int mx = 0;
    for (int i = 0; i < (int)v.size(); i++) mx = max(mx, v[i] + i + 1);

    return mx;
}

int main() {
    ios::sync_with_stdio(false); cin.tie(nullptr);

    int n, a, b;
    cin >> n >> a >> b; a--; b--;

    E.assign(n, {});
    for (int i = 1; i < n; i++) {
        int x, y;
        cin >> x >> y;
        
        E[--x].push_back(--y);
        E[y].push_back(x);
    }

    pr.assign(n, 0);
    dfs_pr(a);

    vector<int> v;

    int cur = pr[b];
    while (cur != -1) {
        v.push_back(cur);
        cur = pr[cur];
    }

    reverse(v.begin(), v.end());

    int l = 0, r = (int)v.size() - 1;
    while (l <= r) {
        int md = (l + r) >> 1;
    
        vis.assign(n, false);

        vis[v[md]] = true;
        int y = dfs(b);

        vis[v[md]] = false;
        int x = dfs(a);

        if (x <= y) l = md + 1;
        else r = md - 1;
    }

    int ans = INT_MAX;
    if (l) {
        vis.assign(n, false);
        
        vis[v[l - 1]] = true;
        ans = dfs(b);
    }
    if (l < (int)v.size()) {
        vis.assign(n, false);

        vis[v[l]] = true;
        int z = dfs(b);

        vis[v[l]] = false;
        ans = min(ans, dfs(a));
    }

    cout << ans;
}

Compilation message (stderr)

torrent.cpp: In function 'int main()':
torrent.cpp:90:13: warning: unused variable 'z' [-Wunused-variable]
   90 |         int z = dfs(b);
      |             ^

• Subtask 1: 31 / 31
• Subtask 2: 69 / 69