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input/code.cpp: In function 'bool dfs(std::vector<long long int>&, std::vector<std::vector<long long int> >&, std::vector<long long int>&)':
input/code.cpp:15:1: warning: no return statement in function returning non-void [-Wreturn-type]
   15 | }
      | ^
input/code.cpp: In function 'int main()':
input/code.cpp:66:23: warning: comparison of integer expressions of different signedness: 'std::vector<long long int>::size_type' {aka 'long unsigned int'} and 'long long int' [-Wsign-compare]
   66 |     if (result.size() != n) {
      |         ~~~~~~~~~~~~~~^~~~
input/code.cpp:71:23: warning: comparison of integer expressions of different signedness: 'long long int' and 'std::vector<long long int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   71 |     for (int i = 0; i < result.size(); i++)
      |                     ~~^~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <unordered_map>
#include <queue>
#include <climits>
 
#define int long long
using namespace std;

bool dfs(vector<int>& res, vector<vector<int>>& adj, vector<int>& state)
{

}

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

    vector<vector<int>> adj(n, vector<int>());

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

    // Vector to store indegree of each vertex
    vector<int> indegree(n);
    for (int i = 0; i < n; i++) {
        for (auto it : adj[i]) {
            indegree[it]++;
        }
    }

    // Queue to store vertices with indegree 0
    queue<int> q;
    for (int i = 0; i < n; i++) {
        if (indegree[i] == 0) {
            q.push(i);
        }
    }
    vector<int> result;
    while (!q.empty()) {
        int node = q.front();
        q.pop();
        result.push_back(node);

        // Decrease indegree of adjacent vertices as the
        // current node is in topological order
        for (auto it : adj[node]) {
            indegree[it]--;

            // If indegree becomes 0, push it to the queue
            if (indegree[it] == 0)
                q.push(it);
        }
    }

    // Check for cycle
    if (result.size() != n) {
        cout << "IMPOSSIBLE";
        return 0;
    }

    for (int i = 0; i < result.size(); i++)
        cout << result[i]+1 << " ";

    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