复制无向带环图

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
         /          \_/

复制无向带环图。

思路：按照深度搜索递归遍历每个顶点，遍历结果是返回以临界点为起始点的子图。要考虑带环的情况，又要避免同一节点访问多次，可用map来判重。

 static class UndirectedGraphNode {
        int label;
        List<UndirectedGraphNode> neighbors;

        UndirectedGraphNode(int x) {
            label = x;
            neighbors = new ArrayList<UndirectedGraphNode>();
        }
    }

    static public UndirectedGraphNode dfs(UndirectedGraphNode node, Map<Integer, UndirectedGraphNode> vis) {
        if (node != null && vis.get(node.label) == null) {

            UndirectedGraphNode cloneNode = new UndirectedGraphNode(node.label);
            vis.put(node.label, cloneNode);
            cloneNode.neighbors = new ArrayList<UndirectedGraphNode>(node.neighbors.size());

            for (int i = 0; i < node.neighbors.size(); i++) {
                cloneNode.neighbors.add(dfs(node.neighbors.get(i), vis));
            }

            return cloneNode;
        } else if (node == null) {
            return node;
        } else {
            //deal if the graph has a circle
            return vis.get(node.label);
        }

    }

    static public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        Map<Integer, UndirectedGraphNode> vis = new HashMap<Integer, UndirectedGraphNode>();
        return dfs(node, vis);

    }

    public static void main(String[] args) {
        UndirectedGraphNode node = new UndirectedGraphNode(0);
        node.neighbors.add(node);
        System.out.println(node.neighbors.get(0) == node);
        UndirectedGraphNode clone = cloneGraph(node);
        System.out.println(clone.label);
    }