You are given the `root` of a binary tree containing digits from `0` to `9` only.

Each root-to-leaf path in the tree represents a number.

- For example, the root-to-leaf path `1 -> 2 -> 3` represents the number `123`.

Return the total sum of all root-to-leaf numbers. Test cases are generated so that the answer will fit in a 32-bit integer.

A leaf node is a node with no children.

Example 1:

    Input: root = [1,2,3]
    Output: 25
    Explanation:
    The root-to-leaf path 1->2 represents the number 12.
    The root-to-leaf path 1->3 represents the number 13.
    Therefore, sum = 12 + 13 = 25.

Example 2:

    Input: root = [4,9,0,5,1]
    Output: 1026
    Explanation:
    The root-to-leaf path 4->9->5 represents the number 495.
    The root-to-leaf path 4->9->1 represents the number 491.
    The root-to-leaf path 4->0 represents the number 40.
    Therefore, sum = 495 + 491 + 40 = 1026.

Constraints:

- The number of nodes in the tree is in the range `[1, 1000]`.
- `0 <= Node.val <= 9`
- The depth of the tree will not exceed `10`.