This is a simple problem; direct use of finding critical point algorithm. Given a graph, you have to find all the critical point(articulation point).
#include <bits/stdc++.h>
using namespace std;
#define mx 1000005
vector<int>grp[mx];
int low[mx], dsc[mx], t;
bool ctp[mx];
bool vst[mx];
int res = 0;
void Bridge(int u, int v) {
// cout<<u<<"-->"<<v<<" "<<"is a bridge"<<endl;
}
void Cutpoint(int u) {
if(ctp[u] == false){
// cout<<u<<" is a cutpoint"<<endl;
res++;
}
ctp[u] = true;
}
void dfs(int sr, int p = -1) {
vst[sr] = true;
t++; low[sr] = dsc[sr] = t;
int ch = 0;
for (int i = 0; i < grp[sr].size(); i++) {
int v = grp[sr][i];
if (v == p) continue;
if (vst[v] == true) {
low[sr] = min(low[sr], dsc[v]);
} else {
dfs(v, sr);
low[sr] = min(low[sr], low[v]);
if (low[v] > dsc[sr]) Bridge(sr, v);
if (low[v] >= dsc[sr] && p != -1) Cutpoint(sr);
ch++;
}
}
if (p == -1 && ch > 1) Cutpoint(sr);
}
int main() {
int n;
while (cin >> n and n) {
int sr;
cin.ignore();
vector<int>v;
while (true) {
string str;
getline(cin, str);
if (str[0] == '0') break;
stringstream s(str);
string word;
while (s >> word)
{
v.emplace_back(stoi(word));
}
if (v[0] == 0) break;
for (int i = 1; i < v.size(); i++) {
grp[v[0]].push_back(v[i]);
grp[v[i]].push_back(v[0]);
sr = v[i];
}v.clear();
}
res = 0;
dfs(sr, -1);
cout << res << endl;
for(int i=0; i<=n; i++){
low[i] = dsc[i] = 0;
ctp[i] = vst[i] = false;
grp[i].clear();
}t = 0;
}
}