How should I practice Sum Root-to-Leaf Numbers?

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Sum Root-to-Leaf Numbers

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Problem statement

You are given the root of a binary tree. Every node stores one decimal digit from 0 through 9. Each root-to-leaf path represents the number formed by reading its digits from root to leaf.

Return the sum of the numbers represented by all root-to-leaf paths. A leaf has no children. Return 0 for an empty tree.

Follow-up

How can an iterative traversal avoid call-stack overflow on a very deep tree?

Function

sumNumbers(root: TreeNode) → int

Examples

Example 1

root = [1,2,3]return = 25

The paths form 12 and 13, whose sum is 25.

Example 2

root = [4,9,0,5,1]return = 1026

The paths form 495, 491, and 40.

Example 3

root = []return = 0

An empty tree has no root-to-leaf paths.

Constraints

Each node value is between 0 and 9.
The tree may be empty.
The final sum fits in a signed 32-bit integer.