[leecode]Clone Graph

Clone Graph

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

OJ's undirected graph serialization:

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 #.

1. First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
2. Second node is labeled as 1. Connect node 1 to node 2.
3. 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
         / 
         \_/

算法分析：

BFS的变种，维护一个map来记录已经生成的copy node，如果已经生成，则直接从map中取就行了，否则需要new出一个新的。

维护一个set来记录在queue中，或者已经出queue的点（其实没有必要用set的，直接可以在queue里查询的，但是考虑到效率，还是用了set，而且只记录label，空间也很省）

代码如下：

public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null) return null;
        Queue<UndirectedGraphNode> q = new LinkedList<UndirectedGraphNode>();
        Set<Integer> set = new HashSet<Integer>();
        Map<Integer,UndirectedGraphNode> map = new HashMap<Integer,UndirectedGraphNode>();
        set.add(node.label);
        q.offer(node);
        map.put(node.label,new UndirectedGraphNode(node.label));
        while(!q.isEmpty()){
            UndirectedGraphNode tem = q.poll();
            UndirectedGraphNode copy = map.get(tem.label);
            List<UndirectedGraphNode> neighbors = tem.neighbors;
            for(UndirectedGraphNode neighbor : neighbors){
                if(!set.contains(neighbor.label)){
                    set.add(neighbor.label);
                    q.offer(neighbor);
                }
                if(!map.containsKey(neighbor.label)){
                    UndirectedGraphNode copyNeighbor = new UndirectedGraphNode(neighbor.label);
                    map.put(neighbor.label,copyNeighbor);
                    copy.neighbors.add(copyNeighbor);
                }else{
                    copy.neighbors.add(map.get(neighbor.label));
                }
            }
        }
        return map.get(node.label);
    }
}