I knew it was a DFS problem pretty quickly but fumbled the accumulation logic at first.
Use a depth-first search (DFS) to traverse the tree, maintaining the current number formed by the path from root to current node. At each leaf, add the current number to a running sum. This approach efficiently computes the sum in O(n) time and O(h) space, where h is the tree height.
Pro tip: Clarify with the interviewer whether the tree can be empty or contain negative digits, and discuss potential integer overflow if the tree is deep. Mention that you can use a 64-bit integer or modular arithmetic if needed.
Confirm that each root-to-leaf path forms a number by concatenating digits, and the goal is to sum all such numbers. Ask about edge cases like empty tree, single node, or negative digits.
Decide between recursive DFS or iterative stack-based DFS. Recursive is simpler but may risk stack overflow for very deep trees; iterative avoids that but is more complex.
Define a helper function that takes a node and the current number formed so far. At each node, update the current number as current * 10 + node.val. If it's a leaf, return the current number; otherwise, return the sum of left and right subtrees.
If the root is null, return 0. If a node has only one child, continue the path without adding a zero. Ensure the algorithm works for negative digits if allowed.
State that time complexity is O(n) since each node is visited once, and space complexity is O(h) for recursion stack. Walk through a small example to verify correctness.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.