Submission #1180516

#	Time	Username	Problem	Language	Result	Execution time	Memory
1180516		LucaIlie	COVID tests (CEOI24_covid)	C++20	40.80 / 100	998 ms	980 KiB

Main

#include <cassert>
#include <cstdio>
#include <string>
#include <vector>
#include <bits/stdc++.h>

using namespace std;

/// You may use:

// The number of students
int N;

// The probability any given student is positive
double P;

// This function performs a test on a subset of samples.
// Its argument is a vector of Booleans of length N,
// where the i-th element is true if the i-th sample should be added to the mix.
// It returns true if (and only if) at least one of the samples in the mix is positive.
bool test_students(std::vector<bool> mask) {
    assert(mask.size() == (size_t)N);

    std::string mask_str(N, ' ');
    for (int i = 0; i < N; i++)
        mask_str[i] = mask[i] ? '1' : '0';

    printf("Q %s\n", mask_str.c_str());
    fflush(stdout);

    char answer;
    scanf(" %c", &answer);
    return answer == 'P';
}

/// You should implement:

// This function will be called once for each test instance.
// It should use test_students to determine which samples are positive.
// It must return a vector of Booleans of length N,
// where the i-th element is true if and only if the i-th sample is positive.

const int MAX_N = 2e3;
double probAllNegative[MAX_N], expectedSteps[MAX_N];
vector<int> cuts[MAX_N];
vector<int> perm;
int expectedPositives;

bool askRange( int l, int r ) {
    vector<bool> mask( N, 0 );
    for ( int i = l; i <= r; i++ )
        mask[perm[i]] = true;
    return test_students( mask );
}

vector<int> positive;
const double CST = 0.5;
void divide( int l, int r ) {
    bool ans = true, asked = false;
    ans &= askRange( l, r );
    asked = true;

    if ( !ans )
        return;

    if ( l == r ) {
        positive.push_back( perm[l] );
        return;
    }

    int i = l;
    for ( int x: cuts[r - l + 1] ) {
        divide( i, i + x - 1 );
        i += x;
    }
    assert( i == r + 1 );
}

const unsigned seed = 100;
const double EPS = 1e-1;
mt19937 rnd( seed );
vector<bool> find_positive() {
    expectedPositives = ceil( N * (P + EPS) );
    positive.clear();
    perm.clear();
    perm.resize( N );
    for ( int i = 0; i < N; i++ )
        perm[i] = i;
    shuffle( perm.begin(), perm.end(), rnd );
    divide( 0, N - 1 );
    vector<bool> answer( N, 0 );
    for ( int i: positive )
        answer[i] = true;

    return answer;
}

int main() {
    int T;
    scanf("%d %lf %d", &N, &P, &T);

    // You may perform any extra initialization here.

    probAllNegative[0] = 1;
    for ( int i = 1; i <= N; i++ ) 
        probAllNegative[i] = probAllNegative[i - 1] * (1 - P);//, cout << i << " " << probAllNegative[i] << "\n";

    expectedSteps[1] = 0;
    cuts[1].push_back( 1 );
    for ( int i = 2; i <= N; i++ ) {
        expectedSteps[i] = 2 * i;
        int bestcut = 1;
        for ( int j = i - 1; j >= 1; j-- ) {
            double newEx = 1 + max( 1.0, expectedSteps[i - j] ) + expectedSteps[j] * (1 - probAllNegative[j] );
            if ( newEx < expectedSteps[i] ) {
                expectedSteps[i] = newEx;
                bestcut = j;
            }
        }

        cuts[i].push_back( bestcut );
        for ( int x: cuts[i - bestcut] )
            cuts[i].push_back( x );

        if ( bestcut <= 3 || bestcut >= i - 3 ) {
            cuts[i].clear();
            cuts[i].push_back( i / 2 );
            cuts[i].push_back( i - i / 2 );
        }
        //printf( "%d %d %lf\n", i, bestcut, expectedSteps[i] ); 
    }

    for (int i = 0; i < T; i++) {
        std::vector<bool> answer = find_positive();
        assert(answer.size() == (size_t)N);

        std::string answer_str(N, ' ');
        for (int j = 0; j < N; j++)
            answer_str[j] = answer[j] ? '1' : '0';

        printf("A %s\n", answer_str.c_str());
        fflush(stdout);

        char verdict;
        scanf(" %c", &verdict);
        if (verdict == 'W')
            exit(0);
    }

    return 0;
}

Compilation message (stderr)

Main.cpp: In function 'bool test_students(std::vector<bool>)':
Main.cpp:32:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   32 |     scanf(" %c", &answer);
      |     ~~~~~^~~~~~~~~~~~~~~~
Main.cpp: In function 'int main()':
Main.cpp:100:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
  100 |     scanf("%d %lf %d", &N, &P, &T);
      |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
Main.cpp:145:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
  145 |         scanf(" %c", &verdict);
      |         ~~~~~^~~~~~~~~~~~~~~~~

• Subtask 0: 0 / 0
• Subtask 1: 10 / 10
• Subtask 2: 30.8 / 90