Sum of Root To Leaf Binary Numbers

The problem. Every node of a binary tree holds a 0 or a 1. Each root-to-leaf path spells a binary number, with the root as the most significant bit. Return the sum of all these numbers. (The answer fits in a 32-bit integer.)

Example

Input:  root = [1, 0, 1, 0, 1, 0, 1]
Output: 22        (100 + 101 + 110 + 111 in binary = 4 + 5 + 6 + 7)

Input:  root = [0]     Output: 0

Reading a binary number left to right works like this: start at 0, and for each new bit, double the number and add the bit. 1 → 1, 10 → 2, 101 → 5. Walking down a path reads its bits in exactly that order.