Verify Preorder Sequence in Binary Search Tree
A medium-tier problem at 51% community acceptance, tagged with Array, Stack, Tree. Reported in interviews at Zenefits and 1 others.
Verify Preorder Sequence in Binary Search Tree is the kind of problem that looks straightforward until you realize the naive approach either times out or misses edge cases. Zenefits and Nvidia have both asked it. You're given an array that's supposed to represent a preorder traversal of a BST, and you need to return whether it's valid. The acceptance rate sits around 51%, which means half the people walking in confident get humbled by the constraints or by missing the key insight that makes it efficient.
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Verify Preorder Sequence in Binary Search Tree is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop.
Get StealthCoderThe trap is building the tree and checking it, which works but is slow. The real pattern is this: in a preorder traversal, you see the root first, then all left subtree nodes (which are smaller), then all right subtree nodes (which are larger). As you iterate through the array, you can maintain a lower bound. When the bound gets violated, you know the sequence is invalid. A monotonic stack lets you track these bounds efficiently. If you hit this problem live and freeze on the tree-building approach, StealthCoder surfaces the stack-based solution in seconds, invisible to the proctor. The algorithm runs in O(n) time instead of O(n^2), which is the difference between passing and timing out.
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Verify Preorder Sequence in Binary Search Tree recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Built by an Amazon engineer who used it to pass JPMorgan's OA and system design loop. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Verify Preorder Sequence in Binary Search Tree interview FAQ
Is this problem still asked at FAANG and mid-tier companies?+
Yes. Zenefits and Nvidia both report asking it. The 51% acceptance rate tells you it's recent and actively used in screens. It's common enough that you'll see it in problem banks but not so common that every candidate has memorized the stack trick.
What's the trick that separates the 51% who pass from those who don't?+
Recognizing that you don't need to build the tree. Instead, track a lower bound as you traverse the preorder array using a monotonic stack. When a value violates the BST property relative to the bound, return false. This keeps you at O(n) instead of O(n^2).
How does this problem relate to the other BST topics?+
It combines BST validation (knowing the ordering rules), recursion (you can solve it that way too, though it's slower), and monotonic stack (the optimal approach). You're really testing whether you understand preorder traversal structure, not just BST properties.
What happens if I just try to build the tree and validate it?+
It works on small inputs but times out on large arrays. The problem is designed to force you past the brute force solution. Large test cases punish O(n^2) approaches, which is why the acceptance rate is capped at 51%.
Do I need to handle null or empty arrays?+
Edge cases matter. An empty array is usually a valid preorder (represents an empty tree). A single element is always valid. Make sure your solution doesn't crash on these before submitting, as they're common gotchas in screening.
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