Given a binary tree, design an algorithm which creates a linked list of all the nodes at each depth (e.g., if you have a tree with depth D, you'll have D linked lists). Return a array containing all the linked lists.

Example:

Input: [1,2,3,4,5,null,7,8]



        1

       /  \ 

      2    3

     / \    \ 

    4   5    7

   /

  8



Output: [[1],[2,3],[4,5,7],[8]]