Submission #1275587

#	Time	Username	Problem	Language	Result	Execution time	Memory
1275587		mehrzad	Migrations (IOI25_migrations)	C++17	28.09 / 100	40 ms	1284 KiB

migrations

#include "migrations.h"
#include <vector>
#include <utility>
#include <algorithm>

using namespace std;

/*
Safe tweaks:
- Reduce the talk window to K=32 (was 50) to lower M without touching correctness.
- Protocol unchanged:
  * Maintain online diameter with bounded LCA.
  * Speak only in the last K edges.
  * At i == startIdx: send A+1
    At i == startIdx+1: send B+1
  * For any later real diameter change in the suffix: send other+1 at that i.
- Museum:
  * startIdx = max(1, N-K)
  * If any message exists at i >= startIdx+2, return (i, S[i]-1).
  * Else return (S[startIdx]-1, S[startIdx+1]-1).
*/

namespace {
    bool inited = false;
    int Nglob = 0, LOGv = 0;
    int startIdx = 1;  // start of last-K window

    // LCA tables
    vector<int> depth;
    vector< vector<int> > up;  // up[v][k], k in [0..LOGv-1]

    // Online diameter state
    int A = 0, B = 0, D = 0;

    // Suffix init flags
    bool sentInitA = false;
    bool sentInitB = false;

    inline void reset_test(int N) {
        inited = true;
        Nglob = N;
        LOGv = 0; while ((1 << LOGv) <= (Nglob > 0 ? Nglob : 1)) ++LOGv;
        depth.assign(Nglob, 0);
        up.assign(Nglob, vector<int>(LOGv, 0));
        for (int k = 0; k < LOGv; ++k) up[0][k] = 0;

        A = B = 0; D = 0;

        // K = 32 (was 50). Keep at least 1 edge in the suffix if N is tiny.
        int K = (Nglob >= 2 ? 32 : max(1, Nglob - 1));
        startIdx = max(1, Nglob - K);

        sentInitA = false;
        sentInitB = false;
    }

    inline int lca(int u, int v) {
        if (depth[u] < depth[v]) swap(u, v);
        int diff = depth[u] - depth[v];
        for (int k = 0; k < LOGv; ++k) if (diff & (1 << k)) u = up[u][k];
        if (u == v) return u;
        for (int k = LOGv - 1; k >= 0; --k) {
            if (up[u][k] != up[v][k]) {
                u = up[u][k];
                v = up[v][k];
            }
        }
        return up[u][0];
    }

    inline int dist(int u, int v) {
        int w = lca(u, v);
        return depth[u] + depth[v] - 2 * depth[w];
    }
}

int send_message(int N, int i, int Pi) {
    // New test starts at i == 1
    if (!inited || i == 1) reset_test(N);

    // Build lifting for node i
    depth[i] = depth[Pi] + 1;
    up[i][0] = Pi;
    for (int k = 1; k < LOGv; ++k) up[i][k] = up[ up[i][k-1] ][k-1];

    // Online diameter update
    int dA = dist(i, A);
    int dB = dist(i, B);
    bool changed = false;
    int other = -1;
    if (dA > D) { changed = true; other = A; B = i; D = dA; }
    else if (dB > D) { changed = true; other = B; A = i; D = dB; }

    int out = 0;

    if (i >= startIdx) {
        if (i == startIdx && !sentInitA) {
            out = A + 1;    // first init
            sentInitA = true;
        } else if (i == startIdx + 1 && !sentInitB) {
            out = B + 1;    // second init
            sentInitB = true;
        } else if (changed) {
            out = other + 1; // real change message
        }
        // else: silent
    }

    return out; // 0 means no message; else 1..10000
}

std::pair<int,int> longest_path(std::vector<int> S) {
    int N = (int)S.size();
    if (N <= 1) return {0, 0};

    // Must match K=32 used in send_message
    int K = (N >= 2 ? 32 : max(1, N - 1));
    int start = max(1, N - K);

    // Look for the last message strictly after the two inits
    for (int i = N - 1; i >= start + 2; --i) {
        if (S[i] != 0) {
            return { i, S[i] - 1 };
        }
    }

    // Otherwise fall back to the two deterministic init messages
    int u = (S[start]     > 0 ? S[start]     - 1 : 0);
    int v = (S[start + 1] > 0 ? S[start + 1] - 1 : 0);
    return { u, v };
}

• Subtask 0: 0 / 0
• Subtask 1: 16 / 30
• Subtask 2: 12.0896 / 70