431. Connected Component in Undirected Graph [LintCode]

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.)

Notice

Each connected component should sort by label.

Clarification

Learn more about representation of graphs

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();
        if(nodes == null)
            return result;

        Map<UndirectedGraphNode, Boolean> visited = new HashMap();
        for(UndirectedGraphNode node : nodes){
            visited.put(node, false);
        }

        for(UndirectedGraphNode node : nodes){
            if(visited.get(node) == false){
                helper(node, visited, result);
            }
        }

        return result;
    }

    private void helper(UndirectedGraphNode node, Map<UndirectedGraphNode, Boolean> visited, 
                            List<List<Integer>> result){

        ArrayList<Integer> row = new ArrayList();
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        queue.offer(node);
        visited.put(node, true);

        while(!queue.isEmpty()){
            UndirectedGraphNode head = queue.poll();
            row.add(head.label);
            for(UndirectedGraphNode nei : head.neighbors){
                if(visited.get(nei) == false){
                    queue.offer(nei);
                    visited.put(nei, true);
                }
            }
        }

        Collections.sort(row);
        result.add(row);
    }
}