DSA · Trees · #173

Sum Root to Leaf Numbers

Module 44 · difficulty 3/5·⏱ 30:00starts on first keystroke

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 formed by concatenating the digits along the path (the root digit is the most significant). Return the **total sum** of all root-to-leaf numbers. A **leaf** is a node with no children. The answer is guaranteed to fit in a 32-bit integer. Implement `sumNumbers(root)` where `root` is the root node (`{ val, left, right }`) or `null`.

Examples

root = [1,2,3] → 25 — Path 1->2 = 12, path 1->3 = 13. 12 + 13 = 25.
root = [4,9,0,5,1] → 1026 — Paths: 4->9->5 = 495, 4->9->1 = 491, 4->0 = 40. 495 + 491 + 40 = 1026.
root = [5] → 5 — A single node is itself a leaf forming the number 5.

Constraints

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

Session phases

A · Clarify

B · Approach

C · Complexity

D · Edges

E · Code

F · Tradeoff

G · Score

Phase A — Clarify

Ask questions about input bounds, types, and edge constraints.

Ask the coach clarifying questions about the problem.

When you've covered this phase, advance to the next.