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input/code.cpp: In function 'int main()':
input/code.cpp:29:19: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
   29 |     for(int i=0; i<m; i++){
      |                  ~^~
input/code.cpp:81:19: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   81 |     for(int i=1; i<order.size(); i++){
      |                  ~^~~~~~~~~~~~~
input/code.cpp:84:19: warning: comparison of integer expressions of different signedness: 'int' and 'unsigned int' [-Wsign-compare]
   84 |     for(int i=1; i<=n; i++){
      |                  ~^~~

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

int p[100001];
map<int, vector<int>> adj;
vector<int> order;

// bool process(int a){
//     if(p[a] == 2) return true;
//     if(p[a] == 1) return false;

//     p[a] = 1;
//     for(auto b : adj[a]){
//         if(!process(b)){
//             return false;
//         }
//     }
//     p[a] = 2;
//     order.push_back(a);
//     return true;
// }

int main(){
    unsigned n, m;
    cin >> n >> m;

    for(int i=0; i<m; i++){
        int a, b;
        cin >> a >> b;
        adj[a].push_back(b);
    }

    order.reserve(n);
    // for(auto&[a,_] : adj){
    //     if(p[a] == 0){
    //         if(!process(a)){
    //             cout << "IMPOSSIBLE" << endl;
    //             return 0;
    //         }
    //     }
    // }

    for(auto&[x,_] : adj){
        if(p[x] == 2)
            continue;
        stack<int> q;
        q.push(x);

        while(!q.empty()){
            int a = q.top();
            // if(p[a] == 1){
            //     p[a] = 2;
            //     q.pop();
            //     continue;
            // }
            p[a] = 1;
            bool ps = true;
            for(auto b : adj[a]){
                if(p[b] == 0){
                    q.push(b);
                    ps = false;
                    break;
                }
                if(p[b] == 1){
                    cout << "IMPOSSIBLE" << endl;
                    return 0;
                }
            }
            if(ps){
                p[a] = 2;
                q.pop();
                order.push_back(a);
            }
        }
    }

    order = vector(order.rbegin(), order.rend());
    cout << order[0];
    for(int i=1; i<order.size(); i++){
        cout << " " << order[i];
    }
    for(int i=1; i<=n; i++){
        if(find(order.begin(), order.end(), i) == order.end()){
            cout << " " << i;
        }
    }
    cout << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED

Test 8

Verdict: TIME LIMIT EXCEEDED

Test 9

Verdict: TIME LIMIT EXCEEDED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED