Find the Connected Component in the Undirected Graph

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.)

Have you met this question in a real interview? 

Yes

Example

Given graph:

A------B  C
 \     |  | 
  \    |  |
   \   |  |
    \  |  |
      D   E

Return {A,B,D}, {C,E}. Since there are two connected component which is{A,B,D}, {C,E}




/**

 * Definition for Undirected graph.

 * class UndirectedGraphNode {

 *     int label;

 *     ArrayList<UndirectedGraphNode> neighbors;

 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }

 * };

 */

public class Solution {

    /**

     * @param nodes a array of Undirected graph node

     * @return a connected set of a Undirected graph

     */

    public List<List<Integer>> connectedSet(ArrayList<UndirectedGraphNode> nodes) {

        // Write your code here

        List<List<Integer>> result = new ArrayList<List<Integer>>();

        List<Integer> path = new ArrayList<Integer>();

        Set<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>();

        for(UndirectedGraphNode node : nodes){

            if(!visited.contains(node)){

                dfs(node, visited, path);

                Collections.sort(path);

                result.add(new ArrayList<Integer>(path));

                path.clear();

            }

        }

        return result;

        

    }

    

    public void dfs(UndirectedGraphNode node, Set<UndirectedGraphNode> visited, List<Integer> path){

        visited.add(node);

        path.add(node.label);

        for(UndirectedGraphNode n : node.neighbors){

            if(!visited.contains(n)){

                dfs(n, visited, path);

            }

        }

    }

}

/**
 * Definition for Undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    /**
     * @param nodes a array of Undirected graph node
     * @return a connected set of a Undirected graph
     */
    public List<List<Integer>> connectedSet(ArrayList<UndirectedGraphNode> nodes) {
        // Write your code here
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        List<Integer> path = new ArrayList<Integer>();
        Set<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>();
        for(UndirectedGraphNode node : nodes){
            if(!visited.contains(node)){
                path.clear();
                Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
                queue.offer(node);
                visited.add(node);
                path.add(node.label);
               while(!queue.isEmpty()){
                        UndirectedGraphNode n = queue.poll();
                        for(UndirectedGraphNode nn : n.neighbors){
                            if(!visited.contains(nn)) {
                                visited.add(nn);
                                queue.offer(nn);
                                path.add(nn.label);
                            }    
                        }
                }
                Collections.sort(path);
                result.add(new ArrayList<Integer>(path));    
            }
               
        }
        return result;
    }

}