Problem: Path Sum II
Problem Description:  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  5   1
return

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

backtracking + recursion