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`

.