Submission #1218898

#	Time	Username	Problem	Language	Result	Execution time	Memory
1218898		j_vdd16	COVID tests (CEOI24_covid)	C++20	66.45 / 100	980 ms	396 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';
}

/*
f(n, p) = 1 + min(
    (1 - (1 - p)^k) * f(n - k, p) +
    (1 - p)^k * f(n, 1/k + (1 - 1/k)*p)
)
*/

/*
1100010010

0001
*/

using namespace std;

void solve(std::vector<bool>& result, int l, int r) {
    bool foundPositive = false;
    int i = l;
    while (i < r) {
        double bestProb = -1.0;
        int bestCount = -1;

        double pow = 1.0 - P;
        int count = 1;
        while (count <= r - i) {
            // allNeg = (r - i - count) / (r - i) * (1 - P)^count
            double pNeg = foundPositive ? pow : (r - i - count) / double(r - i) * pow;
            if (abs(pNeg - 0.5) < abs(bestProb - 0.5)) {
                bestCount = count;
                bestProb = pNeg;
            }

            pow *= 1.0 - P;
            count++;
        }
        
        count = bestCount;
        if (count == r - i && !foundPositive) {
            if (count == 1) {
                result[i] = true;
                return;
            }

            solve(result, i, i + count);
            return;
        }

        std::vector<bool> mask(N);
        for (int j = i; j < i + count; j++) {
            mask[j] = true;
        }

        bool res = test_students(mask);
        if (res && count == 1) {
            foundPositive = true;
            result[i] = true;
        }
        else if (res) {
            foundPositive = true;
            solve(result, i, i + count);
        }

        i += count;
    }
}

/// 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);
    if (P == 0.0)
        return answer;
    if (P == 1.0) {
        for (int i = 0; i < N; i++)
            answer[i] = true;
        
        return answer;
    }

    std::vector<int> students(N);
    for (int i = 0; i < N; i++)
        students[i] = i;

    std::vector<bool> mask(N, true);
    if (!test_students(mask))
        return answer;

    solve(answer, 0, N);

    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:131:10: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
  131 |     scanf("%d %lf %d", &N, &P, &T);
      |     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~
Main.cpp:147:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
  147 |         scanf(" %c", &verdict);
      |         ~~~~~^~~~~~~~~~~~~~~~~

• Subtask 0: 0 / 0
• Subtask 1: 10 / 10
• Subtask 2: 56.45 / 90