133. Clone Graph

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

采用hash map来表示graph。 clone的过程可以通过dfs完成，如果map中已经存在node，则返回该node，否则clone该node。

/**
 * 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) {
        Map<Integer, UndirectedGraphNode> map = new HashMap<>();
        return dfs(node, map);
    }
    
    private UndirectedGraphNode dfs(UndirectedGraphNode node, Map<Integer, UndirectedGraphNode> map) {
        if(node == null) return null;
        if(map.containsKey(node.label)) {
            return map.get(node.label);
        } else {
            UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
            map.put(node.label, clone);
            for(UndirectedGraphNode neighbor : node.neighbors) {
                clone.neighbors.add(dfs(neighbor, map));
            }
            return clone;
        }
    }
}