Given a binary tree, collect a tree's nodes as if you were doing this: Collect and remove all leaves, repeat until the tree is empty. Example:
Given binary tree 

          1
         / \
        2   3
       / \     
      4   5    

  Returns [4, 5, 3], [2], [1]. Explanation: 1. Removing the leaves [4, 5, 3] would result in this tree:

          1
         / 
        2          

  2. Now removing the leaf [2] would result in this tree:

          1          

  3. Now removing the leaf [1] would result in the empty tree:

          []         

    Returns [4, 5, 3], [2], [1].   https://zhuhan0.blogspot.com/2017/05/leetcode-366-find-leaves-of-binary-tree.html

 

 https://www.youtube.com/watch?v=2vwTmHTL1Mk&ab_channel=XavierElon

 

 这题还比较新（对我），解决办法是计算每个node的height，子节点是0，网上走，root = Math。max（left，right）+ 1。root==null就返回-1.

然后看是否需要加入新的ArrayList，然后把相同height的node加到对应的list中