Merge Two Binary Trees
A easy-tier problem at 79% community acceptance, tagged with Tree, Depth-First Search, Breadth-First Search. Reported in interviews at MongoDB and 0 others.
Merge Two Binary Trees is an easy problem that's asked at MongoDB and shows up frequently in real assessments. The acceptance rate is 78%, which sounds forgiving until you realize the gotcha isn't algorithmic, it's handling the merge logic correctly without overwriting or losing nodes. You'll get a tree problem that feels obvious at first, then trips you up on edge cases like null children or asymmetric trees. If you blank on the traversal pattern during your live OA, StealthCoder solves it invisibly in seconds.
Companies that ask "Merge Two Binary Trees"
Merge Two Binary Trees 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 core pattern is simple: walk both trees simultaneously using DFS or BFS, and at each step, merge the node values and recurse on children. The trick is understanding what happens when one tree has a subtree the other doesn't. If tree1 has a left child but tree2 doesn't, that subtree from tree1 stays intact in the result. Most candidates either accidentally drop nodes or create null pointer bugs by not checking both children before recursing. The obvious approach, DFS on both trees in parallel, is correct, but you have to handle the four cases: both nodes exist (merge and recurse), one is null (return the non-null one), both null (return null). BFS works too but adds queue overhead. StealthCoder is your hedge if you freeze on the null-handling logic or mix up which tree's node goes where.
Pattern tags
You know the problem.
Make sure you actually pass it.
Merge Two Binary Trees 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.
Merge Two Binary Trees interview FAQ
Is this problem actually easy or is the 78% acceptance rate misleading?+
It's genuinely easy once you see the pattern. The 78% rate reflects that most people solve it correctly. The catch is interviewer follow-ups: modifying in-place vs. creating new trees, or handling massive trees efficiently. Core solution is straightforward.
Do I need BFS or is DFS fine?+
DFS is simpler and the default choice here. It's recursive, cleaner code, same time and space complexity. BFS works but adds queue boilerplate. Stick with recursive DFS unless the interviewer asks for iterative.
What's the main pitfall interviewers look for?+
Forgetting to handle the case where one tree is null or has a missing subtree. Candidates often assume both trees are symmetric. Also watch your base case: if both nodes are null, return null, not an error.
Does MongoDB actually ask this, or is it a fluke?+
MongoDB is listed as asking it, but with only one company reported, take it as a signal rather than a guarantee. The problem is common enough in the ecosystem that it's worth knowing regardless.
How does this relate to other tree topics?+
It's the simplest application of DFS and BFS on trees. You're not searching or balancing, just traversing two trees in lockstep and combining nodes. Solid foundation before tackling harder tree merge or construction problems.
Want the actual problem statement? View "Merge Two Binary Trees" on LeetCode →