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struct graph
{
	int n;
	vector<vector<int>> adj;
 
	graph(int n) : n(n), adj(n) {}
 
	void add_edge(int u, int v)
	{
		adj[u].push_back(v);
		adj[v].push_back(u);
	}
 
	int add_node()
	{
		adj.push_back({});
		return n++;
	}
 
	vector<int>& operator[](int u) { return adj[u]; }
};
 
vector<vector<int>> biconnected_components(graph &adj)
{
	int n = adj.n;
 
	vector<int> num(n), low(n), art(n), stk;
	vector<vector<int>> comps;
 
	function<void(int, int, int&)> dfs = [&](int u, int p, int &t)
	{
		num[u] = low[u] = ++t;
		stk.push_back(u);
 
		for (int v : adj[u]) if (v != p)
		{
			if (!num[v])
			{
				dfs(v, u, t);
				low[u] = min(low[u], low[v]);
 
				if (low[v] >= num[u])
				{
					art[u] = (num[u] > 1 || num[v] > 2);
 
					comps.push_back({u});
					while (comps.back().back() != v)
						comps.back().push_back(stk.back()), stk.pop_back();
				}
			}
			else low[u] = min(low[u], num[v]);
		}
	};
 
	for (int u = 0, t; u < n; ++u)
		if (!num[u]) dfs(u, -1, t = 0);
 
	// build the block cut tree
	function<graph()> build_tree = [&]()
	{
		graph tree(0);
		vector<int> id(n);
 
		for (int u = 0; u < n; ++u)
			if (art[u]) id[u] = tree.add_node();
 
		for (auto &comp : comps)
		{
			int node = tree.add_node();
			for (int u : comp)
				if (!art[u]) id[u] = node;
				else tree.add_edge(node, id[u]);
		}
 
		return tree;
	};
 
	return comps;
}

prog.cpp:4:2: error: ‘vector’ does not name a type
  vector<vector<int>> adj;
  ^~~~~~
prog.cpp:20:2: error: ‘vector’ does not name a type
  vector<int>& operator[](int u) { return adj[u]; }
  ^~~~~~
prog.cpp: In constructor ‘graph::graph(int)’:
prog.cpp:6:23: error: class ‘graph’ does not have any field named ‘adj’
  graph(int n) : n(n), adj(n) {}
                       ^~~
prog.cpp: In member function ‘void graph::add_edge(int, int)’:
prog.cpp:10:3: error: ‘adj’ was not declared in this scope
   adj[u].push_back(v);
   ^~~
prog.cpp: In member function ‘int graph::add_node()’:
prog.cpp:16:3: error: ‘adj’ was not declared in this scope
   adj.push_back({});
   ^~~
prog.cpp: At global scope:
prog.cpp:23:1: error: ‘vector’ does not name a type
 vector<vector<int>> biconnected_components(graph &adj)
 ^~~~~~

Standard output is empty