Submission #1218242

#	Time	Username	Problem	Language	Result	Execution time	Memory
1218242		j_vdd16	COVID tests (CEOI24_covid)	C++20	34.65 / 100	7058 ms	416 KiB

Main

#include <cassert>
#include <cstdio>
#include <string>
#include <vector>

/// 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';
}

using namespace std;

void solve(std::vector<bool>& result, const std::vector<int>& students, double p) {
    double probAllNegative = 1.0;
    int count = 0;
    while (probAllNegative > 0.5) {
        probAllNegative *= 1.0 - p;
        count++;
    }

    if (0.5 - probAllNegative > probAllNegative / (1.0 - p) - 0.5)
        count--;

    int n = students.size();
    std::vector<int> newStudents;
    for (int i = 0; i < n; i += count) {
        std::vector<bool> mask(N);
        for (int j = i; j < min(n, i + count); j++) {
            mask[students[j]] = true;
        }

        bool res = test_students(mask);
        if (res && count == 1) {
            result[students[i]] = true;
        }
        else if (res) {
            for (int j = i; j < min(n, i + count); j++) {
                newStudents.push_back(students[j]);
            }
        }
    }

    if (newStudents.size()) {
        double pPos = 1.0 / double(count);
        double newP = pPos * 1.0 + (1 - pPos) * p;
        solve(result, newStudents, newP);
    }
}

/// 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.
std::vector<bool> find_positive() {
    std::vector<bool> answer(N);
    std::vector<int> students(N);
    for (int i = 0; i < N; i++)
        students[i] = i;

    solve(answer, students, P);

    return answer;
}

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

    // You may perform any extra initialization here.

    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:29:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   29 |     scanf(" %c", &answer);
      |     ~~~~~^~~~~~~~~~~~~~~~
Main.cpp: In function 'int main()':
Main.cpp:91:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   91 |     scanf("%d %lf %d", &N, &P, &T);
      |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
Main.cpp:107:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
  107 |         scanf(" %c", &verdict);
      |         ~~~~~^~~~~~~~~~~~~~~~~

• Subtask 0: 0 / 0
• Subtask 1: 0 / 10
• Subtask 2: 34.65 / 90