Sum Root to Leaf Numbers

Run a depth-first traversal with current, the number formed by the path before the current node. At a node, compute next = current*10 + node.val. If it is a leaf, return next because that root-to-leaf path forms one complete number. Otherwise return dfs(left, next) + dfs(right, next), treating null children as zero. Passing the number as an argument avoids mutable shared state and makes backtracking unnecessary. The approach assumes node values are decimal digits; if values can exceed nine or the base differs, clarify the encoding. Every node is visited once, so time is O(n), and recursion uses O(h) stack space where h is tree height. Use a sufficiently wide numeric type if depth can make the path number overflow. A common mistake is adding partial prefixes at internal nodes, which counts paths that do not end at leaves.