Longest route

#include <iostream>
#include <vector>
#include <algorithm>
#include <cstring>

using namespace std;

const int N = 1e5 + 5;

int n, m;
vector<int> g[N], ng[N];
int c[N];
int f[N], p[N];
vector<int> ord;

void dfs(int u) {
  c[u] = 1;
  for (int v : g[u]) {
    if (c[v] == 0) {
      dfs(v);
    }
  }
  c[u] = 2;
  ord.emplace_back(u);
}

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int a, b;
    cin >> a >> b;
    g[a].emplace_back(b);
    ng[b].emplace_back(a);
  }
  for (int i = 1; i <= n; i++) {
    if (c[i] == 0) {
      dfs(i);
    }
  }
  reverse(ord.begin(), ord.end());
  memset(f, -1, sizeof(f));
  for (int i : ord) {
    if (i == 1) {
      f[i] = 1;
    } else {
      for (int j : ng[i]) {
        if (1 + f[j] > f[i] && f[j] != -1) {
          f[i] = 1 + f[j];
          p[i] = j;
        }
      }
    }
  }
  if (f[n] == -1) {
    cout << "IMPOSSIBLE" << '\n';
    return 0;
  }
  cout << f[n] << '\n';
  vector<int> ans;
  while (n != 1) {
    ans.emplace_back(n);
    n = p[n];
  }
  ans.emplace_back(1);
  reverse(ans.begin(), ans.end());
  for (int x : ans) {
    cout << x << ' ';
  }
  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

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED