bridges.cpp

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;

template <class T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;

#define int long long int
#define pb push_back
#define pi pair<int, int>
#define pii pair<int, pi>
#define fir first
#define sec second
#define MAXN 400005
#define mod 1000000007

int n, m, timer;
vector<pi> edges;
vector<bool> is_bridge;
vector<pi> adj[MAXN];
int tin[MAXN];
int low[MAXN];
bool vis[MAXN];

void dfs(int v, int p)
{
  vis[v] = true;
  tin[v] = timer, low[v] = timer++;
  for (auto const &u : adj[v])
  {
    if (u.fir == p)
      continue;
    if (vis[u.fir])
    {
      low[v] = min(low[v], tin[u.fir]);
      continue;
    }
    dfs(u.fir, v);
    low[v] = min(low[v], low[u.fir]);
    if (low[u.fir] > tin[v])
      is_bridge[u.sec] = 1;
  }
}
signed main()
{
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n >> m;
  is_bridge.resize(m);
  for (int i = 0; i < m; i++)
  {
    int a, b;
    cin >> a >> b;
    a--, b--;
    edges.pb({a, b});
    adj[a].pb({b, i});
    adj[b].pb({a, i});
  }
  memset(tin, -1, sizeof(tin));
  memset(low, -1, sizeof(low));
  for (int i = 0; i < n; i++)
  {
    if (!vis[i])
      dfs(i, -1);
  }
  return 0;
}