lintcode137- Clone Graph- medium

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 #.

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
     / 
     \_/

Example

return a deep copied graph.

三步走。1. BFS 得到nodes Arraylist  2. copy nodes  3. copy edges

数据结构：map(节点映射)， queue(BFS)+set(BFS时避免加重复)

反正一定得要先2再3，没办法同时做，但你没必要23都各自做一次BFS，可以先1BFS把节点有多少个的信息提取出来，给后面直接用。

另外切记queue.offer()和set.add()一定要同时进行！分开后在重复加某条边的操作下会出错。

1. 自己实现(两次BFS有点冗余了，学习下)

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


public class Solution {
    /*
     * @param node: A undirected graph node
     * @return: A undirected graph node
     */
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        // write your code here
        if (node == null) {
            return null;
        }
        
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<>();
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        queue.offer(node);
        
        while (!queue.isEmpty()) {
            UndirectedGraphNode crt = queue.poll();
            UndirectedGraphNode crtCopy = new UndirectedGraphNode(crt.label);
            map.put(crt, crtCopy);
            for (UndirectedGraphNode neighbor: crt.neighbors) {
                if (!map.containsKey(neighbor)) {
                    queue.offer(neighbor);
                }
            }
        }
        
        Set<UndirectedGraphNode> nodeProcessed = new HashSet<>();
        // queue的offer和set的add一定要同步进行！！！在别的地方add可能会出错处理重复情况
        queue.offer(node);
        nodeProcessed.add(node);
        while (!queue.isEmpty()) {
            UndirectedGraphNode crt = queue.poll();
            UndirectedGraphNode crtCopy = map.get(crt);
            for (UndirectedGraphNode neighbor: crt.neighbors) {
                UndirectedGraphNode neighborCopy = map.get(neighbor);
                crtCopy.neighbors.add(neighborCopy);
                if (!nodeProcessed.contains(neighbor)) {
                    queue.offer(neighbor);
                    nodeProcessed.add(neighbor);
                }
            }
        }
        
        return map.get(node);
    }
}

2.九章实现(额外函数以模块化也值得学习)

public class Solution {
    /**
     * @param node: A undirected graph node
     * @return: A undirected graph node
     */
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) {
            return node;
        }

        // use bfs algorithm to traverse the graph and get all nodes.
        ArrayList<UndirectedGraphNode> nodes = getNodes(node);
        
        // copy nodes, store the old->new mapping information in a hash map
        HashMap<UndirectedGraphNode, UndirectedGraphNode> mapping = new HashMap<>();
        for (UndirectedGraphNode n : nodes) {
            mapping.put(n, new UndirectedGraphNode(n.label));
        }
        
        // copy neighbors(edges)
        for (UndirectedGraphNode n : nodes) {
            UndirectedGraphNode newNode = mapping.get(n);
            for (UndirectedGraphNode neighbor : n.neighbors) {
                UndirectedGraphNode newNeighbor = mapping.get(neighbor);
                newNode.neighbors.add(newNeighbor);
            }
        }
        
        return mapping.get(node);
    }
    
    private ArrayList<UndirectedGraphNode> getNodes(UndirectedGraphNode node) {
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        HashSet<UndirectedGraphNode> set = new HashSet<>();
        
        queue.offer(node);
        set.add(node);
        while (!queue.isEmpty()) {
            UndirectedGraphNode head = queue.poll();
            for (UndirectedGraphNode neighbor : head.neighbors) {
                if(!set.contains(neighbor)){
                    set.add(neighbor);
                    queue.offer(neighbor);
                }
            }
        }
        
        return new ArrayList<UndirectedGraphNode>(set);
    }
}