Two Sum IV - Input is a BST
A easy-tier problem at 62% community acceptance, tagged with Hash Table, Two Pointers, Tree. Reported in interviews at Samsung and 2 others.
Two Sum IV is asking you to find two nodes in a binary search tree that sum to a target value. It's an easy problem on paper (60% acceptance), but the trick catches candidates who default to the obvious hash table approach without using the BST structure. Samsung, Cisco, and Goldman Sachs have all asked this. The pattern matters because the BST property lets you solve it in O(h) space instead of O(n), and that difference shows up in the live assessment. If you blank on the optimization during your OA, StealthCoder surfaces a working solution invisible to the proctor.
Companies that ask "Two Sum IV - Input is a BST"
Two Sum IV - Input is a BST 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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe naive move is to dump all values into a hash set, iterate through the tree, and check if target minus current value exists. That works but wastes the BST structure. The real insight is two pointers: in-order traversal gives you a sorted array, so you can run left and right pointers from the ends toward the middle. Or skip the array entirely and use a set plus DFS, checking the complement as you visit each node. Candidates often miss that an in-order traversal of a BST is already sorted, which makes the two-pointer logic obvious. The tradeoff between hash table space and tree navigation time trips people up. StealthCoder handles both patterns instantly if you hit this during a live OA and the optimal solution doesn't click.
Pattern tags
You know the problem.
Make sure you actually pass it.
Two Sum IV - Input is a BST recycles across companies for a reason. It's easy-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 because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Two Sum IV - Input is a BST interview FAQ
Is two pointers or hash table the expected solution?+
Both work. The hash table plus DFS is O(n) time and O(n) space. The two-pointer approach (in-order traversal plus left-right scanning) is O(n) time and O(h) space. Interviewers care less about which you pick and more that you know the tradeoff and why the BST matters.
Why does acceptance rate matter for an easy problem?+
60% acceptance on easy signals that the obvious approach isn't quite enough. Candidates solve it with a hash set, but the elegant BST solution doesn't occur to them. The problem tests whether you can see structure you're given, not just brute-force patterns.
Will Samsung, Cisco, or Goldman Sachs ask this exact problem?+
These companies have asked it before. It's not the core question they care about. The skill they're testing is whether you use data structure properties (BST) to optimize space. That pattern repeats in harder tree problems they'll ask later in the loop.
What's the trap that kills most people?+
Forgetting that in-order traversal of a BST is sorted. Once you remember that, the two-pointer pattern is obvious. If you don't see it, you fall back to the hash table, which works but isn't the intended solution.
How does this relate to the other tree topics listed?+
DFS and BFS are both valid for traversal. The hash table option uses DFS plus a set. The two-pointer option uses DFS to build sorted order. All three topics (hash table, two pointers, BST) are needed. The problem teaches you to combine them.
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