Longest route

#include <bits/stdc++.h>
using namespace std;

#define int long long
#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

string to_string(string s) {
    return '"' + s + '"';
}
 
string to_string(const char* s) {
    return to_string((string) s);
}
 
string to_string(bool b) {
    return (b ? "true" : "false");
}
 
template <typename A, typename B>
string to_string(pair<A, B> p) {
    return "(" + to_string(p.first) + ", " + to_string(p.second) + ")";
}
 
template <typename A>
string to_string(A v) {
    bool first = true;
    string res = "{";
    for (const auto &x : v) {
        if (!first) {
            res += ", ";
        }
        first = false;
        res += to_string(x);
    }
    res += "}";
    return res;
}
 
void debug_out() {
    cerr << endl;
}
 
template <typename Head, typename... Tail>
void debug_out(Head H, Tail... T) {
    cerr << " " << to_string(H);
    debug_out(T...);
}
 
#ifdef LOCAL
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#else
#define debug(...) 42
#endif

signed main() {
    cin.tie(0)->sync_with_stdio(0);
    cin.exceptions(cin.failbit); // RTE if input wrong datatype

    int n, m;
    cin >> n >> m;

    vector<vector<int>> adj(n), radj(n);
    for (int i = 0; i < m; i++) {
        int u, v;
        cin >> u >> v;
        u--; v--;

        adj[u].push_back(v);
        radj[v].push_back(u);
    }

    vector<int> d(n, -1), p(n, -1);

    queue<int> q;
    d[0] = 0;
    q.push(0);

    while (sz(q)) {
        int u = q.front();
        q.pop();

        for (int v : adj[u]) {
            if (d[v] == -1) {
                d[v] = d[u] + 1;
                p[v] = u;
                q.push(v);
            }
        }
    }

    while (sz(q)) q.pop();

    vector<int> rd(n, -1), rp(n, -1);
    rd[n - 1] = 0;
    q.push(n - 1);

    while (sz(q)) {
        int u = q.front();
        q.pop();

        for (int v : radj[u]) {
            if (rd[v] == -1) {
                rd[v] = rd[u] + 1;
                rp[v] = u;
                q.push(v);
            }
        }
    }

    int best = 0, ans = -1;
    for (int i = 0; i < n; i++) {
        if (d[i] != -1 && rd[i] != -1 && d[i] + rd[i] > best) {
            best = d[i] + rd[i];
            ans = i;
        }
    }

    if (ans == -1) {
        cout << "IMPOSSIBLE";
    } else {
        vector<int> path;
        int u = ans;
        while (u != 0) {
            path.push_back(u);
            u = p[u];
        }
        path.push_back(0);
        reverse(all(path));

        vector<int> rpath;
        u = ans;
        while (u != n - 1) {
            rpath.push_back(u);
            u = rp[u];
        }
        rpath.push_back(n - 1);
        rpath.erase(rpath.begin());

        for (int i : rpath) path.push_back(i);
        cout << sz(path) << '\n';
        for (int i : path) cout << i + 1 << ' ';
    }
}

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: WRONG ANSWER

Test 8

Verdict: WRONG ANSWER

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: WRONG ANSWER

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: ACCEPTED

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED