List of Depths

Given a binary tree, design an algorithm which creates a list of all the nodes at each depth.

If you have a tree with depth D, you will have D lists.

Example:

       4
      / \
     2   6
    / \ / \
   1  3 5  7

Output:
  Depth 1: [4]
  Depth 2: [2, 6]
  Depth 3: [1, 3, 5, 7]

Return a list of lists of node values.