# Graph Deep Copy

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

return a deep copied graph.

## Note

3个步骤：

1. 从1个点找到所有点, 通过BFS，得到所有点的list
2. 复制所有的点, Mapping一下old和new的关系
3. 复制所有的边，找neighbors

## Code

```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 node: A undirected graph node
     * @return: A undirected graph node
     */
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) {
            return node;
        }

        // use bfs algorithm to traverse the graph and get all nodes.
        ArrayList<UndirectedGraphNode> nodes = getNodes(node);

        // copy nodes, store the old->new mapping information in a hash map
        HashMap<UndirectedGraphNode, UndirectedGraphNode> mapping = new HashMap<>();
        for (UndirectedGraphNode n : nodes) {
            mapping.put(n, new UndirectedGraphNode(n.label));
        }

        // copy neighbors(edges)
        for (UndirectedGraphNode n : nodes) {
            UndirectedGraphNode newNode = mapping.get(n);
            for (UndirectedGraphNode neighbor : n.neighbors) {
                UndirectedGraphNode newNeighbor = mapping.get(neighbor);
                newNode.neighbors.add(newNeighbor);
            }
        }

        return mapping.get(node);
    }

    private ArrayList<UndirectedGraphNode> getNodes(UndirectedGraphNode node) {
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        HashSet<UndirectedGraphNode> set = new HashSet<>();

        queue.offer(node);
        set.add(node);
        while (!queue.isEmpty()) {
            UndirectedGraphNode head = queue.poll();
            for (UndirectedGraphNode neighbor : head.neighbors) {
                if(!set.contains(neighbor)){
                    set.add(neighbor);
                    queue.offer(neighbor);
                }
            }
        }

        return new ArrayList<UndirectedGraphNode>(set);
    }
}
```
