source: https://leetcode.com/problems/sum-root-to-leaf-numbers/description/ Sum Root to Leaf Numbers 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 ...