Road blockade

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

typedef long long ll;

int n, m;
vector<vector<pair<int, int>>> adj;
vector<bool> visited;
vector<int> disc;
vector<int> low;
int time_counter = 0;
vector<int> bridges;

void dfs_bridge(int u, int parent_road) {
    visited[u] = true;
    disc[u] = low[u] = ++time_counter;
    for(auto &[v, road] : adj[u]){
        if(road == parent_road) continue;
        if(!visited[v]){
            dfs_bridge(v, road);
            low[u] = min(low[u], low[v]);
            if(low[v] > disc[u]){
                bridges.push_back(road);
            }
        }
        else{
            low[u] = min(low[u], disc[v]);
        }
    }
}

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    adj.assign(n+1, vector<pair<int, int>>());
    vector<pair<int, int>> roads(m+1);
    for(int road=1; road<=m; road++){
        int a, b;
        cin >> a >> b;
        roads[road] = {a, b};
        adj[a].emplace_back(b, road);
        adj[b].emplace_back(a, road);
    }
    visited.assign(n+1, false);
    disc.assign(n+1, 0);
    low.assign(n+1, 0);
    for(int u=1; u<=n; u++){
        if(!visited[u]){
            dfs_bridge(u, -1);
        }
    }
    int comp_id = 0;
    vector<int> comp(n+1, -1);
    vector<bool> is_bridge(m+1, false);
    for(auto road : bridges){
        is_bridge[road] = true;
    }
    // DFS
    function<void(int)> dfs_assign = [&](int u) {
        comp[u] = comp_id;
        for(auto &[v, road] : adj[u]){
            if(is_bridge[road] || comp[v] != -1){
                continue;
            }
            dfs_assign(v);
        }
    };
    for(int u=1; u<=n; u++){
        if(comp[u]==-1){
            dfs_assign(u);
            comp_id++;
        }
    }
    vector<vector<pair<int, int>>> bridge_tree(comp_id, vector<pair<int, int>>());
    for(auto road : bridges){
        int a = roads[road].first;
        int b = roads[road].second;
        int ca = comp[a];
        int cb = comp[b];
        bridge_tree[ca].emplace_back(cb, road);
        bridge_tree[cb].emplace_back(ca, road);
    }
    int comp1 = comp[1];
    int compn = comp[n];
    if(comp1 == compn){
        cout << "0";
        return 0;
    }
    // BFS
    vector<int> parent_bridge(comp_id, -1);
    vector<int> road_to_parent(comp_id, -1);
    queue<int> q;
    q.push(comp1);
    while(!q.empty()){
        int u = q.front(); q.pop();
        for(auto &[v, road] : bridge_tree[u]){
            if(parent_bridge[v]==-1){
                parent_bridge[v] = u;
                road_to_parent[v] = road;
                q.push(v);
                if(v == compn){
                    break;
                }
            }
        }
    }
    vector<int> path_roads;
    int current = compn;
    while(current != comp1){
        if(road_to_parent[current]==-1){
            break;
        }
        path_roads.push_back(road_to_parent[current]);
        current = parent_bridge[current];
    }
    reverse(path_roads.begin(), path_roads.end());
    if(path_roads.empty()){
        cout << "0";
        return 0;
    }
    cout << path_roads[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