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
         / \
         \_/

思路分析：这题考察图的遍历，可以考虑DFS或者BFS对图进行搜索遍历，同时进行图拷贝。下面给出了BFS基于队列的迭代实现解法。比较巧妙的是，需要用到一个HashMap来保存原来的graph的节点和新生成的graph的节点的对应关系，当在原图中BFS遍历当前节点的相邻节点时，要同时在新图对应节点的相邻节点集合中进行相应节点的添加操作。同时这个HashMap的key域还可以当成visited访问标记集合来使用，只添加没有访问过的相邻节点到队列中。每个节点访问一次，时间复杂度和空间复杂度都是O（n）.

AC Code

/**
 * 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) {
        //0923
        if(node == null) return null;
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>(); //key old graph node; value new graph node
        UndirectedGraphNode newGraphNode = new UndirectedGraphNode(node.label);
        newGraphNode.neighbors = new ArrayList<UndirectedGraphNode>();
        if(node.neighbors.isEmpty()) return newGraphNode;
        LinkedList<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        queue.add(node);
        map.put(node, newGraphNode);
        while(!queue.isEmpty()){
            UndirectedGraphNode curNode = queue.poll();
            for(UndirectedGraphNode nnode : curNode.neighbors){
                if(!map.containsKey(nnode)){
                    UndirectedGraphNode copyNode = new UndirectedGraphNode(nnode.label);
                    map.put(nnode, copyNode);
                    queue.add(nnode);
                }
                map.get(curNode).neighbors.add(map.get(nnode));
            }
        }
        return newGraphNode;
    }
}