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 #.
First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
Second node is labeled as 1. Connect node 1 to node 2.
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
/ \
\_/
[分析] 这道题目看似简单,如果对题意和Java引用概念把握不好得话是写不对的。我的实现版本遇到有节点指向自身的case就挂,虽然知道问题所在却不知道如何修改好,看到网友的实现后豁然开朗,让我对引用的理解又加深了。使用HashMap,保存原节点和复制节点的映射,遇到节点指向自身时将该节点自身的引用加入到neighbors中而不要新建一个对象,好像是废话但自己当时怎么也想不到,练习练习再练习!
public class Solution { public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) { if (node == null) return null; UndirectedGraphNode nodeClone = new UndirectedGraphNode(node.label); LinkedList<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>(); HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>(); queue.offer(node); map.put(node, nodeClone); while (!queue.isEmpty()) { UndirectedGraphNode curr = queue.poll(); List<UndirectedGraphNode> neighbors = curr.neighbors; for (UndirectedGraphNode neigh : neighbors) { if (!map.containsKey(neigh)) { UndirectedGraphNode neighClone = new UndirectedGraphNode(neigh.label); map.get(curr).neighbors.add(neighClone); queue.offer(neigh); map.put(neigh, neighClone); } else { map.get(curr).neighbors.add(map.get(neigh)); } } } return nodeClone; } }
错误版本及fail的case
public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) { if (node == null) return null; UndirectedGraphNode clone = new UndirectedGraphNode(node.label); LinkedList<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>(); LinkedList<UndirectedGraphNode> queueClone = new LinkedList<UndirectedGraphNode>(); queue.offer(node); queueClone.offer(clone); HashSet<UndirectedGraphNode> visited = new HashSet<UndirectedGraphNode>(); visited.add(node); while (!queue.isEmpty()) { UndirectedGraphNode curr = queue.poll(); UndirectedGraphNode currClone = queueClone.poll(); for (UndirectedGraphNode neigh : curr.neighbors) { UndirectedGraphNode neighClone = new UndirectedGraphNode(neigh.label); currClone.neighbors.add(neighClone); if (visited.contains(neigh)) continue; queue.offer(neigh); queueClone.offer(neighClone); visited.add(neigh); } } return clone; }