Find the Connected Component in the Undirected Graph

Question

Problem Statement

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.)

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}

题解1 - DFS

Java

/**
* 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) {
if (nodes == null || nodes.size() == 0) return null;

List<List<Integer>> result = new ArrayList<List<Integer>>();
Set<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>();
for (UndirectedGraphNode node : nodes) {
if (visited.contains(node)) continue;
List<Integer> temp = new ArrayList<Integer>();
dfs(node, visited, temp);
Collections.sort(temp);
}

return result;
}

private void dfs(UndirectedGraphNode node,
Set<UndirectedGraphNode> visited,
List<Integer> result) {

// node is not connected, exclude by for iteration
// if (node.neighbors.size() == 0 ) return;
for (UndirectedGraphNode neighbor : node.neighbors) {
if (visited.contains(neighbor)) continue;
dfs(neighbor, visited, result);
}
}
}

题解2 - BFS

Java

/**
* 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) {
if (nodes == null || nodes.size() == 0) return null;

List<List<Integer>> result = new ArrayList<List<Integer>>();
// log visited node before push into queue
Set<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>();
for (UndirectedGraphNode node : nodes) {
if (visited.contains(node)) continue;
List<Integer> row = bfs(node, visited);
}

return result;
}

private List<Integer> bfs(UndirectedGraphNode node,
Set<UndirectedGraphNode> visited) {

List<Integer> row = new ArrayList<Integer>();
q.offer(node);

while (!q.isEmpty()) {
UndirectedGraphNode qNode = q.poll();
for (UndirectedGraphNode neighbor : qNode.neighbors) {
if (visited.contains(neighbor)) continue;
q.offer(neighbor);
}
}

Collections.sort(row);
return row;
}
}