Clone Graph

1/31/2018
Solution:

/**
 * 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;
        // copy nodes
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        Set<UndirectedGraphNode> set = new HashSet<>();
        queue.offer(node);
        set.add(node);
        while (!queue.isEmpty()) {
            UndirectedGraphNode curNode = queue.poll();
            for(UndirectedGraphNode n: curNode.neighbors) {
                if (set.contains(n)) continue;
                queue.offer(n);
                set.add(n);
            }
        }
        
        // map old and new nodes
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        for (UndirectedGraphNode n: set) {
            map.put(n, new UndirectedGraphNode(n.label));
        }
        
        // assign neighbors
        for (UndirectedGraphNode n: set) {
            UndirectedGraphNode newNode = map.get(n);
            for (UndirectedGraphNode neib: n.neighbors) {
                newNode.neighbors.add(map.get(neib));
            }
        }
        
        return map.get(node);
    }
}

思路：
分三步
1. clone nodes, 把所有nodes都找出来存在collection里面，hashset 和arraylist都可以。
2. 建立旧node和新node的联系，为第三步连接做准备，用hashmap。
3. 把每一个新node的每一个neighbor都找出来。根据和它相连的旧node，找出旧node的neighbors，然后通过mapping找到neighbors对应的新node，组成新neighbors赋给当前旧node。