uva 796, sol to find bridges in graph problem

796.cpp

// https://onlinejudge.org/external/7/796.pdf

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

typedef long long ll;

template<typename T>
void out(T x) { cout << x << endl; exit(0); }
#define watch(x) cout << (#x) << " is " << (x) << endl


int n;
vector<set<int>> g;
vector<int> low, tin, viz;
int cloc = 0;

void dfs(int at, int par) {
  viz[at] = 1;
  tin[at] = cloc++;
  low[at] = tin[at];
  for (int to: g[at]) {
    if (to == par) continue;
    if (viz[to]) {
      low[at] = min(low[at], tin[to]);
    } else {
      dfs(to, at);
      low[at] = min(low[at], low[to]);
    }
  }
}

int main() {
  ios_base::sync_with_stdio(false); cin.tie(0);  cout.tie(0);


  while (scanf("%d", &n) == 1) {
    g = {};
    g.resize(n);
    for (int i=0; i<n; i++) {
      int u; int d; int v;
      scanf("%d ", &u);
      scanf("(%d)", &d);
      while (d--) {
	scanf("%d", &v);
	g[u].insert(v);
	g[v].insert(u);
      }
    }

    low = {}; tin = {}; viz = {};
    low.resize(n); tin.resize(n); viz.resize(n);
    cloc = 0;

    for (int i=0; i<n; i++) {
      if (viz[i] == 0) {
	dfs(i,-1);
      }
    }

    // for (int i=0; i<n; i++) {
    //   cout<<i<<" tin: "<<tin[i]<<" low: "<<low[i]<<endl;
    // }

    set<pair<int,int>> bridges;
    for (int i=0; i<n; i++) {
      for (int j=0; j<n; j++) {
	if (g[i].count(j) == 0 || g[j].count(i) == 0) continue;
	if (low[j] > tin[i]) {
	  int x = min(i,j); int y = max(i,j);
	  bridges.insert({x,y});
	}
      }
    }
    printf("%d critical links\n", (int)bridges.size());
    for (auto ed: bridges) {
      printf("%d - %d\n", ed.first, ed.second);
    }
    printf("\n");
  }
  return 0;
}