原题链接在这里：

题目：

We are given a binary tree (with root node root), a target node, and an integer value K.

Return a list of the values of all nodes that have a distance K from the target node.  The answer can be returned in any order.

 

Example 1:

Input: root = [3,5,1,6,2,0,8,null,null,7,4], target = 5, K = 2Output: [7,4,1]Explanation: The nodes that are a distance 2 from the target node (with value 5)have values 7, 4, and 1.Note that the inputs "root" and "target" are actually TreeNodes.The descriptions of the inputs above are just serializations of these objects.

Note:

1. The given tree is non-empty.
2. Each node in the tree has unique values 0 <= node.val <= 500.
3. The target node is a node in the tree.
4. 0 <= K <= 1000.

题解：

Clone this tree with new type of Node containing a pointer to parent.

Find the cloned target node and do BFS from it.

Time Complexity: O(n). Deep clone takes O(n). BFS takes O(n).

Time Complexity: O(n). Regardless res.

AC Java:

1 /** 2  * Definition for a binary tree node. 3  * public class TreeNode { 4  *     int val; 5  *     TreeNode left; 6  *     TreeNode right; 7  *     TreeNode(int x) { val = x; } 8  * } 9  */10 class Solution {11     Node clonedTargetNode;12     public List
     
       distanceK(TreeNode root, TreeNode target, int K) {13         List
      
        res = new ArrayList
       
        ();14         if(root == null){15             return res;16         }17         18         deepClone(root, null, target);19         LinkedList
        
          que = new LinkedList
         
          ();20         que.add(clonedTargetNode);21         22         HashSet
          
            used = new HashSet
           
            ();23 used.add(clonedTargetNode);24 while(!que.isEmpty()){25 int size = que.size();26 if(K==0){27 for(int i = 0; i