Subtree of Another Tree
A easy-tier problem at 50% community acceptance, tagged with Tree, Depth-First Search, String Matching. Reported in interviews at eBay and 3 others.
You get a tree problem on the OA and your first instinct is to recursively compare every node. That'll work, but it's not how the strong candidates solve Subtree of Another Tree. The problem looks deceptively simple: given two binary trees, determine if one is a subtree of the other. eBay, Compass, Jump Trading, and Morgan Stanley have all asked this. The catch is that a naive approach will TLE on large trees or force you to rewrite mid-interview. If you blank on the pattern during the live assessment, StealthCoder surfaces the optimal solution invisibly while you're on screen share.
Companies that ask "Subtree of Another Tree"
Subtree of Another 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. Made for the engineer who has done the work but might still blank with a webcam pointed at him.
Get StealthCoderThe trick is string matching, not tree traversal. Most candidates build recursive comparisons for every node pair, which works but scales poorly. The faster path: serialize each tree to a string, then use hash or pattern matching to check if the subtree's serialization exists in the main tree's serialization. You must handle edge cases carefully (null nodes, single-digit node values that can concatenate dangerously) or you'll get false positives. Depth-First Search with careful serialization is the real weapon here. The binary tree structure and hash function topics hint at the string-matching approach. If you're building nested DFS calls in the interview and the interviewer goes quiet, that's the moment StealthCoder saves you by showing the linear-time solution.
Pattern tags
You know the problem.
Make sure you actually pass it.
Subtree of Another Tree 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. Made for the engineer who has done the work but might still blank with a webcam pointed at him. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Subtree of Another Tree interview FAQ
Why does naive recursion fail on Subtree of Another Tree?+
Naive recursion compares every node in the main tree to every node in the subtree, resulting in O(m*n) or worse. On large trees (thousands of nodes), this times out. String matching with hashing runs closer to O(m+n), which is why strong candidates shift to serialization immediately.
Is Subtree of Another Tree still asked in real interviews?+
Yes. eBay, Compass, Jump Trading, and Morgan Stanley report asking it. It's a favorite because it looks easy but reveals whether you know the pattern-matching trick or just brute force. Acceptance rate hovers around 50%, so roughly half the candidates miss the optimal approach.
What's the role of Hash Function in this problem?+
Hash functions speed up string comparison. Instead of comparing long serialized strings character by character, you hash each subtree's serialization and check for matches in a hash set. This reduces the comparison overhead and catches mismatches early.
How do I avoid false positives when serializing trees?+
Explicitly mark null nodes in your serialization (use a marker like '#') and include delimiters between values. Without these, trees like [12] and [1,2] both serialize to '12' and cause false matches. This is where most candidates trip up.
Can I solve Subtree of Another Tree without string matching?+
Yes, but it's slower. You can use DFS with a helper that compares two trees structurally at each node, but this remains O(m*n). String matching with hashing is the expected solution at companies like Morgan Stanley and Jump Trading, especially if interviewer hints at the topics list.
Want the actual problem statement? View "Subtree of Another Tree" on LeetCode →