Problem:
Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum.

For example:
Given the below binary tree and sum = 22,

      5
/ \
4   8
/   / \
11  13  4
/  \      \
7    2      1


return

[
[5,4,11,2],
[5,8,4,5]
]