I knew DFS was the move pretty quickly, carry a running number down each path and when you hit a leaf you add it to the total.
Use a depth-first search (DFS) traversal, passing the current accumulated number down the tree. At each leaf, add the accumulated number to a running sum. This avoids storing all paths and computes the sum in O(n) time.
Pro tip: Clarify the problem constraints upfront (e.g., tree size, digit range) and discuss potential integer overflow, suggesting a modulo or big integer handling if needed. Also, mention that an iterative DFS can avoid recursion depth issues for skewed trees.
Restate the problem to confirm understanding: each root-to-leaf path forms a number; sum all such numbers. Ask clarifying questions about tree size, digit values, and expected output type.
Select DFS (preorder) to build numbers incrementally. Explain why DFS is suitable: it naturally maintains the path from root to current node.
Design a helper function that takes a node and the current accumulated value. At each node, update the value as current * 10 + node.val. If leaf, return the value; else, recurse on children and sum results.
Consider an empty tree (return 0), a single node (return its value), and potential integer overflow for deep trees. Discuss using modulo or larger data types if necessary.
State time complexity O(n) and space complexity O(h) for recursion stack. Walk through a small example to verify correctness.
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