Clone Graph

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.

How we serialize an undirected graph:

Nodes are labeled uniquely.

We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.

As an example, consider the serialized graph {0,1,2#1,2#2,2}.

The graph has a total of three nodes, and therefore contains three parts as separated by #.

First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
Second node is labeled as 1. Connect node 1 to node 2.
Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.
Visually, the graph looks like the following:

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    /**
     * @param node: A undirected graph node
     * @return: A undirected graph node
     */
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) {
            return null;
        }
        //使用BFS获取所有的点
        ArrayList<UndirectedGraphNode> nodes = bfsHelper(node);
        //copy 点
        HashMap<UndirectedGraphNode, UndirectedGraphNode> mapping = new HashMap<>();
        for (UndirectedGraphNode n : nodes) {
            mapping.put(n, new UndirectedGraphNode(n.label));
        }
        //copy 边
        for (UndirectedGraphNode n : nodes) {
            //取出原节点对应的新节点
            UndirectedGraphNode newNode = mapping.get(n);
            for (UndirectedGraphNode neighbor : n.neighbors) {
                //取出原节点的邻接点对应的新节点的邻接点
                UndirectedGraphNode newNeighbor = mapping.get(neighbor);
                //将其赋值给新节点的邻接点
                newNode.neighbors.add(newNeighbor);
            }
        }
        return mapping.get(node);
    }
    //图的BFS使用Set来保证每个节点遍历一次和边无关
    private ArrayList<UndirectedGraphNode> bfsHelper(UndirectedGraphNode node) {
        ArrayList<UndirectedGraphNode> nodes = new ArrayList<>();
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        Set<Integer> hash = new HashSet<>();
        queue.offer(node);
        hash.add(node.label);
        while (!queue.isEmpty()) {
            UndirectedGraphNode temp = queue.poll();
            nodes.add(temp);
            for (UndirectedGraphNode neighbor : temp.neighbors) {
                if (hash.contains(neighbor.label)) {
                    continue;
                }
                queue.offer(neighbor);
                hash.add(neighbor.label);
            }
        }
        return nodes;
    }
}