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.

Solution 1: Iterative DFS. Use a stack to store current path sum.

Solution 2: Recursive DFS.

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