Submission #846510

#	Time	Username	Problem	Language	Result	Execution time	Memory
846510		voi	Torrent (COI16_torrent)	C++17	100 / 100	271 ms	26072 KiB

torrent

#include <bits/stdc++.h>
#define pll pair <long long, long long>
#define fi first
#define se second

using namespace std;
#define task "code"
#define all(s) s.begin(), s.end()

typedef long long ll;

const int ar = 3e5+2;
const ll mod = 998244353;
const int oo = 1e9;

int n, a, b, child[ar], dp[ar], pre[ar], suf[ar], f[ar], res = oo;
vector <int> g[ar], trace;

void caltrace(int u, int p) {
    if(trace.empty() || trace.back() != b) {
        trace.push_back(u);
    }
    for(auto v : g[u]) if(v != p)
        caltrace(v, u);
    if(trace.back() != b)
        trace.pop_back();
}

void dfs(int u, int p) {
    vector <int> tg;
    dp[u] = 0;
    for(auto v : g[u]) if(v != p && v != child[u]) {
        dfs(v, u);
//        if(u == 3) cout << v << " 3333\n";
            tg.push_back(dp[v]);
    }
    sort(all(tg), greater<int>());
//    if(u == 3) cout << tg.size() << '\n';
    for(int i = 0; i < tg.size(); ++i)
        dp[u] = max(dp[u], tg[i] + i + 1);
//    cout << u << ' ' << dp[u] << '\n';
}

bool incr(int i) {
    child[trace[i - 1]] = trace[i];
    child[trace[i]] = trace[i - 1];

    dfs(a, 0);
    dfs(b, 0);
    res = min(res, max(dp[a], dp[b]));

    child[trace[i - 1]] = child[trace[i]] = 0;
    return dp[a] < dp[b];
}

int main() {
    ios_base::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    if(fopen(task".inp", "r")) {
        freopen(task".inp", "r", stdin);
        freopen(task".out", "w", stdout);
    }

    cin >> n >> a >> b;
    for(int i = 1; i < n; ++i) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    caltrace(a, 0);
    if(a == b) return cout << dp[a], 0;
    int m = trace.size();
//    for(auto x : trace) cout << x << ' '; cout << '\n';

    int l = 1, r = m - 1;
    while(l <= r) {
        int mid = l + r >> 1;
        if(incr(mid)) l = mid + 1;
        else r = mid - 1;
    }
    cout << res;

    return 0;
}

Compilation message (stderr)

torrent.cpp: In function 'void dfs(int, int)':
torrent.cpp:39:22: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   39 |     for(int i = 0; i < tg.size(); ++i)
      |                    ~~^~~~~~~~~~~
torrent.cpp: In function 'int main()':
torrent.cpp:78:21: warning: suggest parentheses around '+' inside '>>' [-Wparentheses]
   78 |         int mid = l + r >> 1;
      |                   ~~^~~
torrent.cpp:60:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   60 |         freopen(task".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
torrent.cpp:61:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   61 |         freopen(task".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

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