Snakeless path

input/code.cpp: In function 'int main()':
input/code.cpp:87:23: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   87 |         for(int i=0; i<path.size(); ++i){
      |                      ~^~~~~~~~~~~~
input/code.cpp:88:35: warning: comparison of integer expressions of different signedness: 'int' and 'std::vector<int>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   88 |             cout << path[i] << (i < path.size()-1 ? ' ' : '\n');
      |                                 ~~^~~~~~~~~~~~~~~

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

int main(){
    ios::sync_with_stdio(false);
    cin.tie(NULL);
    
    int n, m;
    cin >> n >> m;
    
    // Store blocked edges as sorted vectors
    vector<vector<int>> blocked(n+1, vector<int>());
    
    for(int i=0; i<m; ++i){
        int a, b;
        cin >> a >> b;
        blocked[a].push_back(b);
        blocked[b].push_back(a);
    }
    
    // Sort the blocked lists for each node
    for(int i=1; i<=n; ++i){
        sort(blocked[i].begin(), blocked[i].end());
    }
    
    // Initialize BFS
    vector<int> parent(n+1, -1);
    queue<int> q;
    // Initialize unvisited set with all nodes except 1
    set<int> unvisited;
    for(int i=2; i<=n; ++i){
        unvisited.insert(i);
    }
    
    // Start BFS from node 1
    q.push(1);
    parent[1] = 0;
    
    while(!q.empty()){
        int current = q.front();
        q.pop();
        
        // If reached node n, stop
        if(current == n){
            break;
        }
        
        // Iterate through unvisited nodes and find allowed nodes
        // Allowed nodes are those not in blocked[current]
        // Since both blocked[current] and unvisited are sorted, use two pointers
        auto it = unvisited.begin();
        auto blk_it = blocked[current].begin();
        while(it != unvisited.end()){
            // Advance blk_it to the first blocked node >= *it
            while(blk_it != blocked[current].end() && *blk_it < *it){
                ++blk_it;
            }
            if(blk_it != blocked[current].end() && *blk_it == *it){
                // This node is blocked, skip
                ++it;
            }
            else{
                // This node is allowed
                int node = *it;
                parent[node] = current;
                q.push(node);
                // Erase the node from unvisited and get the next iterator
                it = unvisited.erase(it);
            }
        }
    }
    
    if(parent[n] == -1){
        cout << "0\n";
    }
    else{
        // Reconstruct the path
        vector<int> path;
        int node = n;
        while(node != 0){
            path.push_back(node);
            node = parent[node];
        }
        reverse(path.begin(), path.end());
        // Output
        cout << path.size() << "\n";
        for(int i=0; i<path.size(); ++i){
            cout << path[i] << (i < path.size()-1 ? ' ' : '\n');
        }
    }
}

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: ACCEPTED

Test 71

Verdict: ACCEPTED

Test 72

Verdict: ACCEPTED

Test 73

Verdict: ACCEPTED

Test 74

Verdict: ACCEPTED

Test 75

Verdict: ACCEPTED

Test 76

Verdict: ACCEPTED

Test 77

Verdict: ACCEPTED

Test 78

Verdict: ACCEPTED

Test 79

Verdict: ACCEPTED

Test 80

Verdict: ACCEPTED

Test 81

Verdict: ACCEPTED

Test 82

Verdict: ACCEPTED

Test 83

Verdict: ACCEPTED

Test 84

Verdict: ACCEPTED

Test 85

Verdict: ACCEPTED

Test 86

Verdict: ACCEPTED

Test 87

Verdict: ACCEPTED

Test 88

Verdict: ACCEPTED

Test 89

Verdict: ACCEPTED