Sum Root To Leaf Numbers

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.