133. Clone Graph

    /*
     * 133. Clone Graph
     * 12.19 by Mingyang
     * 第一种方法就是我们用的那种BFS的方法，queue进行广度遍历
     * 不要忘了新建neighbor的点后要注意把关系也copy过来
     * 第二种方法就是DFS的方法，也就是stack来做深度遍历，换汤不换药
     * 无论怎么换，记住，用map比较好，可以记住两个node之间的对应关系
     * 最容易错误的地方在于queue.add(neighbor);必须在if判断句里面，不然会形成loop，只有第一次出现才可以
     */
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode root) {
        if (root == null)
            return null;
        Map<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();
        queue.add(root);
        map.put(root, new UndirectedGraphNode(root.label));
        while (!queue.isEmpty()) {
            UndirectedGraphNode node = queue.poll();
            // handle the neighbors
            for (UndirectedGraphNode neighbor : node.neighbors) {
                if (!map.containsKey(neighbor)) {
                    map.put(neighbor, new UndirectedGraphNode(neighbor.label)); 
      // 这就是一个映射的影子，对于每一个元素，在地图中注册下你的影子
      //这里的queue必须放在if里面加
                    queue.add(neighbor);
                }
                // copy the neighbor
                map.get(node).neighbors.add(map.get(neighbor));
     // 当注册好影子以后，我们需要把原图中的关系也copy过来
            }
        }

        return map.get(root);
    }
    // 用非递归的DFS来做，也就是用Stack
    public UndirectedGraphNode cloneGraph2(UndirectedGraphNode root) {
        if (root == null)
            return null;
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        LinkedList<UndirectedGraphNode> stack = new LinkedList<UndirectedGraphNode>();
        UndirectedGraphNode head = new UndirectedGraphNode(root.label);
        map.put(root, head);
        stack.push(root);
        while (!stack.isEmpty()) {
            UndirectedGraphNode node = stack.pop();
            for (UndirectedGraphNode neighbor : node.neighbors) {
                if (!map.containsKey(neighbor)) {    
                    map.put(neighbor, new UndirectedGraphNode(neighbor.label));
                    stack.push(neighbor);
                }
                map.get(node).neighbors.add(map.get(neighbor));
            }
        }
        return head;
    }    
    //用递归的DFS
    public UndirectedGraphNode cloneGraph3(UndirectedGraphNode node) {
        if(node == null)
            return null;       
        HashMap<UndirectedGraphNode, UndirectedGraphNode> map = new HashMap<UndirectedGraphNode, UndirectedGraphNode>();
        UndirectedGraphNode head = new UndirectedGraphNode(node.label);
        map.put(node, new UndirectedGraphNode(node.label));   
        dfs(map, node);//DFS
        return head;
    }
    public void dfs(HashMap<UndirectedGraphNode, UndirectedGraphNode> map, UndirectedGraphNode node){
        if(node == null)
            return;          
        for(UndirectedGraphNode neighbor: node.neighbors){ 
            if(!map.containsKey(neighbor)){
                map.put(neighbor,  new UndirectedGraphNode(neighbor.label));
                dfs(map, neighbor);//DFS
            }
            map.get(node).neighbors.add(map.get(neighbor));
        }
    }