第一步加到map里面value是null只是為了去重，下一次遇到就不會進(jìn)queue

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null){
            return null;
        }        
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        Map<UndirectedGraphNode, UndirectedGraphNode> visited = new HashMap<>();
        queue.offer(node);
        visited.put(node, null);
        while (!queue.isEmpty()){
            UndirectedGraphNode curt = queue.poll();
            UndirectedGraphNode copy = new UndirectedGraphNode(curt.label);
            visited.put(curt, copy);
            for (UndirectedGraphNode nei : curt.neighbors){
                if (!visited.containsKey(nei)){
                    queue.offer(nei);
                    visited.put(nei, null);
                }
            }
        }
        for (UndirectedGraphNode orig : visited.keySet()){
            UndirectedGraphNode copy = visited.get(orig);
            for (UndirectedGraphNode nei : orig.neighbors){
                copy.neighbors.add(visited.get(nei));
            }
        }
        return visited.get(node);
    }
}