You are given the root of a binary tree where each node contains a single digit from 0 to 9. Each root-to-leaf path represents a number obtained by concatenating the digits along the path from the root down to a leaf. For example, if a path is root → 1 → 2 → 3, it represents the number 123. Write a function that returns the sum of all numbers formed by all root-to-leaf paths in the tree. - Input: the root node of a binary tree. - Output: an integer representing the sum of all root-to-leaf numbers. Constraints: - Each node value is a digit from 0 to 9. - The tree has at most 1000 nodes. Describe your algorithm and its time and space complexity. You do not need to provide actual code.