Question:
Given a binary tree containing digits from 0-9 only,
each root-to-leaf path could represent a number.
An example is the root-to-leaf path 1->2->3 which represents
the number 123.
Find the total sum of all root-to-leaf numbers.
For example,
1 / 2 3
The root-to-leaf path 1->2 represents the number 12.
The root-to-leaf path 1->3 represents the number 13.
Return the sum = 12 + 13 = 25.