Snakeless path

input/code.cpp: In function 'int main()':
input/code.cpp:50:27: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   50 |         for (int i = 0; i < lst.size(); i++) {
      |                         ~~^~~~~~~~~~~~

#include <bits/stdc++.h>

using namespace std;
#define ll long long

int main() {

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

    unordered_set<ll> s;
    for (int i = 0; i < m; i++) {
        ll u, v;
        cin >> u >> v;
        s.insert(((u-1) << 20LL) + (v-1));
        s.insert(((v-1) << 20LL) + (u-1));
    }

    unordered_set<ll> unaccessed;
    for (int i = 1; i < n; i++) unaccessed.insert(i);

    queue<ll> q;
    q.push(0);
    vector<int> pred(n, -1);

    while (unaccessed.count(n-1) > 0 && !q.empty()) {
        ll u = q.front();
        q.pop();
        for (auto it = unaccessed.begin(); it != unaccessed.end();) {
            ll v = *it;
            if (s.count((u<<20) + v) == 0) {
                pred[v] = u;
                q.push(v);
                it = unaccessed.erase(it);
            } else ++it;
        }
    }

    if (unaccessed.count(n-1) == 0) {
        vector<int> lst;
        int c = n-1;
        while (c != 0) {
            lst.push_back(c);
            c = pred[c];
        }
        lst.push_back(0);
        reverse(lst.begin(), lst.end());

        cout << lst.size() << endl;
        for (int i = 0; i < lst.size(); i++) {
            if (i > 0) cout << " ";
            cout << lst[i] + 1;
        }
        cout << endl;
    } else {
        cout << "IMPOSSIBLE" << endl;
    }




}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: WRONG ANSWER

Test 7

Verdict: WRONG ANSWER

Test 8

Verdict: ACCEPTED

Test 9

Verdict: WRONG ANSWER

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: WRONG ANSWER

Test 15

Verdict: WRONG ANSWER

Test 16

Verdict: ACCEPTED

Test 17

Verdict: WRONG ANSWER

Test 18

Verdict: WRONG ANSWER

Test 19

Verdict: WRONG ANSWER

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: WRONG ANSWER

Test 25

Verdict: WRONG ANSWER

Test 26

Verdict: ACCEPTED

Test 27

Verdict: WRONG ANSWER

Test 28

Verdict: WRONG ANSWER

Test 29

Verdict: WRONG ANSWER

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED

Test 33

Verdict: ACCEPTED

Test 34

Verdict: WRONG ANSWER

Test 35

Verdict: WRONG ANSWER

Test 36

Verdict: ACCEPTED

Test 37

Verdict: WRONG ANSWER

Test 38

Verdict: WRONG ANSWER

Test 39

Verdict: WRONG ANSWER

Test 40

Verdict: ACCEPTED

Test 41

Verdict: ACCEPTED

Test 42

Verdict: ACCEPTED

Test 43

Verdict: ACCEPTED

Test 44

Verdict: WRONG ANSWER

Test 45

Verdict: WRONG ANSWER

Test 46

Verdict: ACCEPTED

Test 47

Verdict: WRONG ANSWER

Test 48

Verdict: WRONG ANSWER

Test 49

Verdict: WRONG ANSWER

Test 50

Verdict: ACCEPTED

Test 51

Verdict: ACCEPTED

Test 52

Verdict: ACCEPTED

Test 53

Verdict: ACCEPTED

Test 54

Verdict: ACCEPTED

Test 55

Verdict: WRONG ANSWER

Test 56

Verdict: ACCEPTED

Test 57

Verdict: WRONG ANSWER

Test 58

Verdict: ACCEPTED

Test 59

Verdict: WRONG ANSWER

Test 60

Verdict: ACCEPTED

Test 61

Verdict: WRONG ANSWER

Test 62

Verdict: WRONG ANSWER

Test 63

Verdict: ACCEPTED

Test 64

Verdict: ACCEPTED

Test 65

Verdict: ACCEPTED

Test 66

Verdict: WRONG ANSWER

Test 67

Verdict: WRONG ANSWER

Test 68

Verdict: ACCEPTED

Test 69

Verdict: WRONG ANSWER

Test 70

Verdict: ACCEPTED

Test 71

Verdict: ACCEPTED

Test 72

Verdict: ACCEPTED

Test 73

Verdict: ACCEPTED

Test 74

Verdict: ACCEPTED

Test 75

Verdict: WRONG ANSWER

Test 76

Verdict: ACCEPTED

Test 77

Verdict: WRONG ANSWER

Test 78

Verdict: ACCEPTED

Test 79

Verdict: WRONG ANSWER

Test 80

Verdict: ACCEPTED

Test 81

Verdict: WRONG ANSWER

Test 82

Verdict: WRONG ANSWER

Test 83

Verdict: ACCEPTED

Test 84

Verdict: ACCEPTED

Test 85

Verdict: ACCEPTED

Test 86

Verdict: WRONG ANSWER

Test 87

Verdict: WRONG ANSWER

Test 88

Verdict: ACCEPTED

Test 89

Verdict: WRONG ANSWER