Find the number connected component in the undirected graph. Each node in the graph contains a label and a list of its neighbors. (a connected component (or just component) of an undirected graph is a subgraph in which any two vertices are connected to each other by paths, and which is connected to no additional vertices in the supergraph.)

Solution
BFS + Hashmap -------- get all nodes by BFS, record visited by hashmap

public class Solution {
    /**
     * @param nodes a array of Undirected graph node
     * @return a connected set of a Undirected graph
     */
     
     
     

     //优化点------boolea[] visited instead of arraylist.contains()
    public List<List<Integer>> connectedSet(ArrayList<UndirectedGraphNode> nodes) {
       int m = nodes.size();
        Map<UndirectedGraphNode, Boolean> visited = new HashMap<>();
        
       for (UndirectedGraphNode node : nodes){
            visited.put(node, false);
       }
        
        List<List<Integer>> result = new ArrayList<>();
        
        for (UndirectedGraphNode node : nodes){
            if (visited.get(node) == false){
                bfs(node, visited, result);
            }
        }
        
        return result;
    }
    
     public void bfs(UndirectedGraphNode node, Map<UndirectedGraphNode, Boolean> visited, List<List<Integer>> result){
        List<Integer>row = new ArrayList<>();
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        visited.put(node, true);
        queue.offer(node);
        while (!queue.isEmpty()){
            UndirectedGraphNode u = queue.poll();
            row.add(u.label);    
            for (UndirectedGraphNode v : u.neighbors){
                if (visited.get(v) == false){
                    visited.put(v, true);
                    queue.offer(v);
                }
            }
        }
        Collections.sort(row);
        result.add(row);
        
    }
}