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 #.
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 / \ \_/
哈希表法
复杂度
时间 O(N) 空间 O(N)
思路
广度优先搜索,同时用一个哈希表,将新旧节点映射起来。这样我们第一次遍历到的节点,我们会新建一个节点并映射到哈希表中。当以后再遍历到这个节点时,我们可以直接用哈希表取出它对应的新节点。我们只要保证,对于第一次遇到的图节点,我们都会建立一个克隆节点,并在哈希表映射起来就行了。
注意
这里我们并不需要维护一个visited的集合,为什么呢?因为每次我们遇到一个之前没见过图节点,我们都会给它建立一个克隆节点,然后在哈希表中映射起来,并把这个图节点也放入队列中。所以只要哈希表中有这个图节点,就说明我们之前已经将该图节点放入队列了,就不需要再处理了。不过还是要把该图节点的克隆节点放入父克隆节点的邻居中。
代码
public class Solution {
public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
if(node == null) return null;
Queue<UndirectedGraphNode> q = new LinkedList<UndirectedGraphNode>();
Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
UndirectedGraphNode root = new UndirectedGraphNode(node.label);
map.put(node, root);
q.offer(node);
while(!q.isEmpty()){
UndirectedGraphNode curr = q.poll();
// 将curr旧节点的邻居节点都加入curr的新节点
for(UndirectedGraphNode oldNeighbor : curr.neighbors){
// 判断是否已经生成过该邻居节点的新节点
if(!map.containsKey(oldNeighbor)){
// 如果是第一次生成该新节点,将其加入队列中
map.put(oldNeighbor, new UndirectedGraphNode(oldNeighbor.label));
q.offer(oldNeighbor);
}
// 将新邻居加入新curr节点的neighbors中
map.get(curr).neighbors.add(map.get(oldNeighbor));
}
}
return root;
}
}
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