[LeetCode] 133. Clone Graph Java

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
         / \
         \_/

---

 题意及分析：复制一个无向图。这里考察的是图的遍历，可以使用bfs或者dfs。主要需要使用一个hashmap来映射原图和新图，这样就可以知道某个点是否遍历过。bfs是先遍历某个点，然后遍历该点的所有相邻点，dfs是先遍历某个点，然后向带点的某个相邻点继续遍历。

代码(BFS)：

/**
 * 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;
        Queue<UndirectedGraphNode> queue = new LinkedList<>();
        HashMap<UndirectedGraphNode,UndirectedGraphNode> map = new HashMap<UndirectedGraphNode,UndirectedGraphNode>();  //记录该点是否被遍历过
        UndirectedGraphNode newHead = new UndirectedGraphNode(node.label);
        queue.add(node);
        map.put(node,newHead);

        while(!queue.isEmpty()){
            UndirectedGraphNode cur = queue.poll();
            List<UndirectedGraphNode> curNeighbors=cur.neighbors;
            for(UndirectedGraphNode aNeighbor : curNeighbors){　　　　//遍历该点的所有相邻点
                if(!map.containsKey(aNeighbor)){
                    UndirectedGraphNode copy = new UndirectedGraphNode(　　　　//若该点没有遍历过那么将该点添加进去
                            aNeighbor.label);
                    map.put(aNeighbor,copy);
                    map.get(cur).neighbors.add(copy);
                    queue.add(aNeighbor);
                }else{　　　　//该点已被遍历过，那么将当前点作为该点的相邻点添加进去
                    map.get(cur).neighbors.add(map.get(aNeighbor));
                }
            }
        }
        return newHead;
    }
}

 代码（dfs）：

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    HashMap<UndirectedGraphNode,UndirectedGraphNode> map=new HashMap<>();
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node==null) return null;
        if(map.containsKey(node)){
            return map.get(node);
        }
        UndirectedGraphNode newHead = new UndirectedGraphNode(node.label);
        map.put(node,newHead);
        for(UndirectedGraphNode aNeighbor:node.neighbors){
            newHead.neighbors.add(cloneGraph(aNeighbor));
        }
        return newHead;
    }
}