Submission #1053456

#	Time	Username	Problem	Language	Result	Execution time	Memory
1053456		ilovveyyou	Mousetrap (CEOI17_mousetrap)	C++14	45 / 100	486 ms	81496 KiB

mousetrap

#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second
#define ll long long
#define pll pair<ll, ll>
#define pii pair<int, int>
#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 clz __builtin_clzll
#define ctz __builtin_ctzll
#define popcount __builtin_popcount
#define lg(x) (63 - clz(x))
#define max_rng *max_element
#define min_rng *min_element

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());
    }

const ll oo = (ll) 1e18, inf = (ll) 1e9, mod = (ll) 1e9 + 7;
const int mxn = (int) 1e6 + 5, S = (int) 450, S2 = (int) 450, lg = (int) 18;

void add(ll &x, ll y) {
    x += y;
    if (x >= mod) x -= mod;
}
void sub(ll &x, ll y) {
    x -= y;
    if (x < 0) x += mod;
}
int n, t, m;
vector<int> g[mxn];

int timer = 0;
int in[mxn], out[mxn];

int par[mxn], dist[mxn], h[mxn];

void dfs(int u, int p) {
    in[u] = ++timer;
    if (u != t) {
        for (int v : g[u]) if (v != p)
            dist[u]++;
    }
    for (int v : g[u]) if (v != p) {
        dist[v] += dist[u];
        h[v] = h[u] + 1;
        dfs(v, u);
        par[v] = u;
    }
    out[u] = timer;
}
bool inRoot(int r, int u) {
    return in[r] <= in[u] && in[u] <= out[r];
}

int f[mxn], ban[mxn];
int dp(int u, int p, int blocked) {

    if (f[u] != -1) return f[u];

    int adj = 0;
    for (int v : g[u]) if (v != p && !ban[v]) 
        adj++;

    if (adj <= blocked + 1) {
        return f[u] = adj;
    }

    f[u] = inf;
    int sz = g[u].size();
    vector<int> k(sz, -inf), h(sz, -inf);

    for (int i = 0; i < sz; ++i) {
        int v = g[u][i];
        if (v != p && !ban[v])
            k[i] = h[i] = dp(v, u, blocked) + adj;
    }
    for (int i = 1; i < sz; ++i) {
        maximize(k[i], k[i-1]);
    }
    for (int i = sz-2; i >= 0; --i) {
        maximize(h[i], h[i+1]);
    }

    // cout << "PHASE BEGIN u = " << u << ' ' << p << '\n';
    // for (int i = 0; i < sz; ++i) {
    //     int v = g[u][i];
    //     if (v != p && !ban[v]) {
    //         cout << i << ' ' << v << ' ' << k[i] << ' ' << h[i] << '\n';
    //     }
    // }

    for (int i = 0; i < sz; ++i) {
        int v = g[u][i];
        if (v == p || ban[v]) continue;
        int cur = -inf;
        if (i > 0) maximize(cur, k[i-1]);
        if (i+1 < sz) maximize(cur, h[i+1]);
        if (cur != -inf) minimize(f[u], cur);
    }
    return f[u];
}

int backtrack(int cur, int blocked = 0) {
    if (cur == t) return -inf;
    int res = dp(cur, par[cur], blocked);
    res += dist[par[cur]] - h[par[cur]];
    // cout << cur << ' ' << par[cur] << ' ' << blocked << ' ' << f[cur] << ' ' << dist[par[cur]] - h[par[cur]] << "\n\n\n";
    ban[cur] = true; f[cur] = 0;
    
    return max(res, backtrack(par[cur], blocked+1));
}

void solve() {
    cin >> n >> t >> m;
    for (int i = 2; i <= n; ++i) {
        int u, v;
        cin >> u >> v;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    if (t == m) {
        cout << 0 << '\n';
        return;
    }

    memset(f, -1, sizeof f);

    dfs(t, 0);

    ll ans = backtrack(m);
    assert(ans < inf);

    cout << ans;

    return;
}

int main() {

    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);
    }

    int t = 1;
    // cin >> t;
    while (t--) {
        solve();
    }

    // cerr << "\nTime elapsed: " << (1.0 * clock() / CLOCKS_PER_SEC) << " s.";

    return 0;
}

Compilation message (stderr)

mousetrap.cpp: In function 'int main()':
mousetrap.cpp:162:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  162 |         freopen(TASK".inp", "r", stdin);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
mousetrap.cpp:163:16: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
  163 |         freopen(TASK".out", "w", stdout);
      |         ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

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