Convert Binary Tree to Linked Lists by Depth

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).

Example

Given binary tree:

    1
   / \
  2   3
 /
4

return

[
  1->null,
  2->3->null,
  4->null
]

Output : 

1 ->  null
2 -> 3 ->  null
4 ->  null

Output