Path Sum IV
A medium-tier problem at 63% community acceptance, tagged with Array, Hash Table, Tree. Reported in interviews at Alibaba and 0 others.
Path Sum IV is the kind of medium that looks straightforward on the surface but punishes you if you misread the input format. Alibaba has asked this one. You're given an array where each element encodes a node's depth, position, and value in a single integer. Most candidates waste time trying to build a full tree structure or misparse the encoding, then run out of time. The trick is understanding that you don't need to construct anything. You just need to decode the integers correctly and use DFS to traverse the implicit tree. If you hit this in your live assessment and blank on the encoding scheme, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Path Sum IV"
Path Sum IV 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 engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoderThe problem encodes tree nodes as three-digit integers: first digit is depth (1-4), second is position at that depth (1-9), third is the node value. Most candidates try to build an actual tree or miss that position determines parent-child relationships. The real pattern is simple once you decode it: a node at depth d and position p has children at depth d+1 and positions 2p and 2p+1. This is the same binary tree indexing used in array-based heaps. You decode all nodes into a hash map, then DFS from root (depth 1, position 1) to find all root-to-leaf paths and sum them. The acceptance rate sits at 63 percent because the encoding confuses people at first glance, not because the algorithm is hard. If you practice tree encoding once, you'll never stumble on this again. StealthCoder is your hedge if the encoding format catches you off guard during the OA.
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Path Sum IV 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 engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Path Sum IV interview FAQ
How do I parse the three-digit encoding?+
Each number encodes depth (hundreds place), position (tens place), and value (ones place). Extract them with integer division: depth = num // 100, position = (num // 10) % 10, value = num % 10. A node at (depth, position) has children at (depth+1, 2*position) and (depth+1, 2*position+1).
Do I need to build a tree object?+
No. Parse the input into a hash map keyed by (depth, position), then DFS recursively by computing child coordinates. This avoids node/pointer overhead and is faster. You only need the map and a recursive function.
What's a common mistake on this problem?+
Misunderstanding the encoding scheme or trying to construct a real tree. Some candidates also confuse the position index, or forget that a leaf is a node with no children in the input (not necessarily depth 4). Always check if children exist in the map before recursing.
Why is the acceptance rate only 63 percent for a medium?+
The three-digit encoding format is unusual and throws people off immediately. Once you decode it and recognize the binary indexing pattern, the DFS is straightforward. Most rejections happen in the parsing phase, not the traversal logic.
Is this asked at other companies besides Alibaba?+
The data shows Alibaba as the primary source. It's a LeetCode medium that appears less often in broader interview pipelines, so it's less common than classics like path sum or inorder traversal. Still worth knowing before a big-tech OA.
Want the actual problem statement? View "Path Sum IV" on LeetCode →