Snakeless path

#include <bits/stdc++.h>

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);

    int n, m;
    std::cin >> n >> m;

    std::vector free_path(n, std::vector (n, true));
    while (m--) {
        int a, b;
        std::cin >> a >> b;
        free_path[a-1][b-1] = false;
        free_path[b-1][a-1] = false;
    }

    std::vector<std::vector<int>> graph(n + 1);
    for (int a = 1; a <= n; a++) {
        for (int b = 1; b <= n; b++) {
            if (free_path[a-1][b-1]) {
                graph[a].emplace_back(b);
            }
        }
    }

    std::vector parent(n + 1, 0);
    std::vector visited(n + 1, false);
    visited[1] = true;

    std::queue<int> q;
    q.push(1);
    while (!q.empty() && !visited[n]) {
        const int s = q.front(); q.pop();

        for (auto& v : graph[s]) {
            if (visited[v]) continue;
            parent[v] = s;
            visited[v] = true;
            if (v == n) {
                break;
            }
            q.push(v);
        }
    }

    if (!visited[n]) {
        std::cout << 0 << std::endl;
        return 0;
    }

    std::vector<int> path;
    for (int v = n; v != 0; v = parent[v]) {
        path.emplace_back(v);
    }

    std::cout << path.size() << '\n';
    std::ranges::reverse(path);
    for (auto& v : path) {
        std::cout << v << ' ';
    }
    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

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED

Test 33

Verdict: ACCEPTED

Test 34

Verdict: ACCEPTED

Test 35

Verdict: ACCEPTED

Test 36

Verdict: ACCEPTED

Test 37

Verdict: ACCEPTED

Test 38

Verdict: ACCEPTED

Test 39

Verdict: ACCEPTED

Test 40

Verdict: ACCEPTED

Test 41

Verdict: ACCEPTED

Test 42

Verdict: ACCEPTED

Test 43

Verdict: ACCEPTED

Test 44

Verdict: ACCEPTED

Test 45

Verdict: ACCEPTED

Test 46

Verdict: ACCEPTED

Test 47

Verdict: ACCEPTED

Test 48

Verdict: ACCEPTED

Test 49

Verdict: ACCEPTED

Test 50

Verdict: ACCEPTED

Test 51

Verdict: ACCEPTED

Test 52

Verdict: ACCEPTED

Test 53

Verdict: ACCEPTED

Test 54

Verdict: ACCEPTED

Test 55

Verdict: ACCEPTED

Test 56

Verdict: ACCEPTED

Test 57

Verdict: ACCEPTED

Test 58

Verdict: ACCEPTED

Test 59

Verdict: ACCEPTED

Test 60

Verdict: ACCEPTED

Test 61

Verdict: ACCEPTED

Test 62

Verdict: ACCEPTED

Test 63

Verdict: ACCEPTED

Test 64

Verdict: ACCEPTED

Test 65

Verdict: ACCEPTED

Test 66

Verdict: ACCEPTED

Test 67

Verdict: ACCEPTED

Test 68

Verdict: ACCEPTED

Test 69

Verdict: ACCEPTED

Test 70

Verdict: RUNTIME ERROR

Test 71

Verdict: RUNTIME ERROR

Test 72

Verdict: RUNTIME ERROR

Test 73

Verdict: RUNTIME ERROR

Test 74

Verdict: RUNTIME ERROR

Test 75

Verdict: RUNTIME ERROR

Test 76

Verdict: RUNTIME ERROR

Test 77

Verdict: RUNTIME ERROR

Test 78

Verdict: RUNTIME ERROR

Test 79

Verdict: RUNTIME ERROR

Test 80

Verdict: RUNTIME ERROR

Test 81

Verdict: RUNTIME ERROR

Test 82

Verdict: RUNTIME ERROR

Test 83

Verdict: RUNTIME ERROR

Test 84

Verdict: RUNTIME ERROR

Test 85

Verdict: RUNTIME ERROR

Test 86

Verdict: RUNTIME ERROR

Test 87

Verdict: RUNTIME ERROR

Test 88

Verdict: RUNTIME ERROR

Test 89

Verdict: RUNTIME ERROR