Clone Graph

之所以把这道题单独拿出来，是因为通过它我们可以了解到图的结构，以及如何处理，我们分别用递归，广搜和深搜来完成这道题。

Clone Graph
复制一个无向图，图中每个节点都有一个label和一个neighbors集合。

解决图的题，因为图中存在环，我们要判断哪些点已经访问过了，做上标记，以防止进入死循环。这里我们用哈希函数来判断一个顶点是否被访问过，如果没被访问过就加入到哈希表中。
首先我们通过广搜来完成它。广搜用到队列，代码如下：

/**
 * 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) {
        HashMap<UndirectedGraphNode, UndirectedGraphNode> hm = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        if(node == null) return null;
        UndirectedGraphNode copy = new UndirectedGraphNode(node.label);
        hm.put(node,copy);
        queue.offer(node);
        while(!queue.isEmpty()) {
            UndirectedGraphNode cur = queue.poll();
            for(UndirectedGraphNode neighbor : cur.neighbors) {
                if(!hm.containsKey(neighbor)) {
                    copy = new UndirectedGraphNode(neighbor.label);
                    hm.put(neighbor, copy);
                    queue.offer(neighbor);
                }
                hm.get(cur).neighbors.add(hm.get(neighbor));
            }
        }
        return hm.get(node);
    }
}

深度搜索用到堆栈，代码如下：

/**
 * 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) {
        HashMap<UndirectedGraphNode, UndirectedGraphNode> hm = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        Stack<UndirectedGraphNode> stack = new Stack<UndirectedGraphNode>();
        if(node == null) return null;
        stack.push(node);
        UndirectedGraphNode copy = new UndirectedGraphNode(node.label);
        hm.put(node,copy);
        while(!stack.isEmpty()) {
            UndirectedGraphNode cur = stack.pop();
            for(UndirectedGraphNode neighbor : cur.neighbors) {
                if(!hm.containsKey(neighbor)) {
                    copy = new UndirectedGraphNode(neighbor.label);
                    hm.put(neighbor, copy);
                    stack.push(neighbor);
                }
                hm.get(cur).neighbors.add(hm.get(neighbor));
            }
        }
        return hm.get(node);
    }
}

递归实现：

/**
 * 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) {
        HashMap<UndirectedGraphNode, UndirectedGraphNode> hm = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        return DFS(hm, node);
    }
    private UndirectedGraphNode DFS(HashMap<UndirectedGraphNode, UndirectedGraphNode> hm, UndirectedGraphNode node) {
        if(node == null) return null;
        UndirectedGraphNode copy = new UndirectedGraphNode(node.label);
        hm.put(node, copy);
        for(UndirectedGraphNode neighbor : node.neighbors) {
            if(!hm.containsKey(neighbor)) {
                UndirectedGraphNode neighborcopy = DFS(hm, neighbor);
            }
            copy.neighbors.add(hm.get(neighbor));
        }
        return copy;
    }
}