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input/code.cpp: In function 'int main(int, char**)':
input/code.cpp:24:20: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   24 |     for (int i=0; i<a.size(); i++) {
      |                   ~^~~~~~~~~
input/code.cpp:34:18: warning: comparison of integer expressions of different signedness: 'std::vector<int>::size_type' {aka 'long unsigned int'} and 'int' [-Wsign-compare]
   34 |     if (a.size() != n) {
      |         ~~~~~~~~~^~~~

#include <iostream>
#include <vector>


int main(int argc, char **argv) {
    int n, q;
    std::cin >> n >> q;

    std::vector<std::vector<int>> opens(n+1);
    std::vector<int> precomputed_opens(n+1, 0);
    for (int i=1; i<=q; i++) {
        int query, cell;
        std::cin >> query >> cell;
        opens[query].push_back(cell);
        precomputed_opens[cell] = precomputed_opens[cell] + 1;
    }

    std::vector<int> a;
    for (int i=1; i<=n; i++) {
        if (precomputed_opens[i] == 0)
            a.push_back(i);
    }

    for (int i=0; i<a.size(); i++) {
        int query = a[i];

        for (int cell : opens[query]) {
            precomputed_opens[cell] = precomputed_opens[cell] - 1;
            if (precomputed_opens[cell] == 0)
                a.push_back(cell);
        }
    }

    if (a.size() != n) {
        std::cout << "IMPOSSIBLE" << std::endl;
        return 0;
    }

    for (int b : a) {
        std::cout << b << " ";
    }
    std::cout << std::endl;

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED