You are given the root of a binary tree where each node has a value 0 or 1. Each root-to-leaf path represents a binary number starting with the most significant bit. For example, if the path is 0 -> 1 -> 1 -> 0 -> 1, then this could represent 01101

sumRootToLeaf.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */
var sumRootToLeaf = function(root) {
    // DFS helper: carries the current binary value down the tree
    function dfs(node, currentValue) {
        if (!node) {
            // Null nodes contribute nothing
            return 0;
        }

        // Build the new binary number by shifting left and adding the current bit
        const nextValue = (currentValue << 1) + node.val;

        // If this is a leaf, we've completed a full root-to-leaf binary number
        if (!node.left && !node.right) {
            return nextValue;
        }

        // Otherwise, explore both children and sum their contributions
        return dfs(node.left, nextValue) + dfs(node.right, nextValue);
    }

    // Start DFS with initial value 0
    return dfs(root, 0);
};