133. Clone Graph

题目：

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
         / 
         \_/

链接： http://leetcode.com/problems/clone-graph/

5/14/2017

14ms, 9%

用BFS的方法，先建立每个元素的copy，然后如果neighbor没有建立copy，把neighbor也放在queue当中。第二部分就是链接各个节点。

还要注意iterate through hashmap

/**
 * 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) {
        if (node == null) return null;
        
        Map<UndirectedGraphNode, UndirectedGraphNode> graphMap = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        
        queue.offer(node);
        // create copy of new nodes
        while (!queue.isEmpty()) {
            UndirectedGraphNode n = queue.poll();
            if (!graphMap.containsKey(n)) {
                UndirectedGraphNode copy = new UndirectedGraphNode(n.label);
                graphMap.put(n, copy);
            }
            for (UndirectedGraphNode neighbor: n.neighbors) {
                if (!graphMap.containsKey(neighbor)) {
                    queue.offer(neighbor);
                }
            }
        }
        // build neighbors
        for (Map.Entry<UndirectedGraphNode, UndirectedGraphNode> entry: graphMap.entrySet()) {
            UndirectedGraphNode original = entry.getKey();
            UndirectedGraphNode copy = entry.getValue();
            for (UndirectedGraphNode neighbor: original.neighbors) {
                copy.neighbors.add(graphMap.get(neighbor));
            }
        }
        return graphMap.get(node);
    }
}

也可以在copy的时候直接建立neighbors

https://discuss.leetcode.com/topic/4690/simple-java-iterative-bfs-solution-with-hashmap-and-queue

DFS做法

https://discuss.leetcode.com/topic/9629/depth-first-simple-java-solution

更多讨论

https://discuss.leetcode.com/category/141/clone-graph