Copy Graph.java

/*
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:

   1
  / \
 /   \
0 --- 2
     / \
     \_/

time = O(V+E)
space = O(V) for the map and queue
*/

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     ArrayList<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */


public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // time = o(V + E)
        // space = O(V)
        if (node == null) {
            return null;
        }
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        queue.offer(node);
        map.put(node, new UndirectedGraphNode(node.label));
        
        while (!queue.isEmpty()) {
            UndirectedGraphNode cur = queue.poll();
            UndirectedGraphNode newCur = map.get(cur);
            for (UndirectedGraphNode neighbor : cur.neighbors) {
                if (!map.containsKey(neighbor)) {
                    queue.offer(neighbor);
                    map.put(neighbor, new UndirectedGraphNode(neighbor.label));
                }
                newCur.neighbors.add(map.get(neighbor));
            }
        }
        return map.get(node);
    }
}