Submission #1087986

#	Time	Username	Problem	Language	Result	Execution time	Memory
1087986		anhthi	Torrent (COI16_torrent)	C++14	100 / 100	452 ms	56808 KiB

torrent

#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second
#define ll long long
#define mp(x, y) make_pair(x, y)
#define sz(v) ((int) (v).size())
#define all(v) (v).begin(), (v).end()
#define MASK(i) (1LL << (i))
#define BIT(x, y) (((x) >> (y)) & 1)
#define pb push_back
#define max_rng *max_element
#define min_rng *min_element
#define rep(i, n) for(int i = 1, _n = (n); i <= _n; ++i)
#define forn(i, a, b) for(int i = (a), _b = (b); i <= _b; ++i)
#define ford(i, a, b) for(int i = (a), _b = (b); i >= _b; --i)

template <class X, class Y>
    inline bool maximize(X &x, Y y) {
        return (x < y ? x = y, true : false);
    }

template <class X, class Y>
    inline bool minimize(X &x, Y y) {
        return (x > y ? x = y, true : false);
    }

template <class X>
    inline void compress(vector<X> &a) {
        sort(all(a));
        a.resize(unique(all(a)) - a.begin());
    }

int ctz(ll x) { return __builtin_ctzll(x); }
int lg(ll x) { return 63 - __builtin_clzll(x); }
int popcount(ll x) { return __builtin_popcount(x); }

mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rand(ll l, ll r) {
    return l + abs((ll) rng()) % (r - l + 1);
}

const ll oo = (ll) 1e17;
const int inf = (ll) 2e9 + 7, mod = (ll) 1e9 + 7;

const int mxn = 3e5 + 5, mxm = 3e2 + 5;
const int LG = 18, BASE = 311;

void add(int &x, int y) { x += y; if (x >= mod) x -= mod; }
void sub(int &x, int y) { x -= y; if (x < 0) x += mod; }

int n, a, b;
vector<int> g[mxn];

int par[mxn];
void calc(int u, int p) {
    par[u] = p;
    for (int v : g[u]) if (v != p)
        calc(v, u);
    return;
}
vector<pair<int, int>> road;

vector<int> g1[mxn], g2[mxn];
void build(int u, int p, int ban, int t) {
    for (int v : g[u]) if (v != p && v != ban) {
        build(v, u, ban, t);
        if (t == 1) {
            g1[v].pb(u);
            g1[u].pb(v);
        }
        else {
            g2[v].pb(u);
            g2[u].pb(v);
        }
    }
}
int f[4][mxn];
void dfs(int u, int p, int t) {
    const vector<int> &adj = (t == 1 ? g1[u] : g2[u]);
    vector<int> lst;
    for (int v : adj) if (v != p) {
        dfs(v, u, t);
        lst.pb(f[t][v]);
    }
    sort(all(lst));
    reverse(all(lst));

    rep(i, sz(lst)) {
        maximize(f[t][u], lst[i-1] + i);
    }
    return;
}

pair<int, int> solve(int pos) {

    build(a, 0, road[pos].fi, 1);
    build(b, 0, road[pos].se, 2);

    rep(i, 2) memset(f[i], 0, sizeof f[i]);
    dfs(a, 0, 1); dfs(b, 0, 2);

    rep(i, n) {
        g1[i].clear();
        g2[i].clear();
    }

    return mp(f[1][a], f[2][b]);
}

int main(void) {

    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);

    #define TASK "task"
    if (fopen(TASK".inp", "r")) {
        freopen(TASK".inp", "r", stdin);
        freopen(TASK".out", "w", stdout);
    }

    cin >> n >> a >> b;
    rep(i, n - 1) {
        int u, v;
        cin >> u >> v;
        g[u].pb(v);
        g[v].pb(u);
    }
    calc(a, 0);

    for (int p = b; p && p != a; p = par[p]) {
        road.pb(mp(p, par[p]));
    }
    int ans = inf;
    for(int l = 0, r = sz(road) - 1; l <= r; ) {
        int m = (l + r) >> 1;
        pair<int, int> res = solve(m);

        if (res.fi >= res.se) {
            l = m + 1;
        }
        else r = m - 1;

        minimize(ans, max(res.fi, res.se));
    }

    cout << ans;

    return 0;
}

Compilation message (stderr)

torrent.cpp: In function 'int main()':
torrent.cpp:120:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  120 |         freopen(TASK".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
torrent.cpp:121:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  121 |         freopen(TASK".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

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