[LeetCode] 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
         / \
         \_/ 

解题思路：

无向图的邻居，要注意避免重复建设node. 所以我们用一个hashmap来存储已经有的点。当遇到了已经有的点，直接从hahmap里取。

Java Code:

 public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
       
        if(node == null){
            return null;
        }
        
        HashMap allnodes = new HashMap(); 
        List newnodes= new ArrayList();
        List oldnodes= new ArrayList();
       
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        allnodes.put(node.label, newNode);
        
        newnodes.add(newNode);
        oldnodes.add(node);
        
      do{  
          List newNext= new ArrayList();
          List oldNext= new ArrayList();
        
          for(int i = 0; i < oldnodes.size(); i++){
              UndirectedGraphNode oldnode = oldnodes.get(i);
            
            if(oldnode.neighbors == null){
                continue;
            }
            
            List oldneibors = oldnode.neighbors;
            List newneibors = new ArrayList();
            
            for(UndirectedGraphNode curr : oldneibors){
                UndirectedGraphNode newnode = null;
                if(allnodes.containsKey(curr.label)){
                   newnode = allnodes.get(curr.label);
                }else{
                  newnode = new UndirectedGraphNode(curr.label);
                  allnodes.put(curr.label, newnode);
                  newNext.add(newnode);
                  oldNext.add(curr);
                }
                newneibors.add(newnode);
            }                  
            newnodes.get(i).neighbors = newneibors;
        }
        
        oldnodes = oldNext;
        newnodes = newNext;
      }while(!oldnodes.isEmpty());
      
      return newNode;
    }