Binary Tree Cameras
A hard-tier problem at 47% community acceptance, tagged with Dynamic Programming, Tree, Depth-First Search. Reported in interviews at DE Shaw and 8 others.
Binary Tree Cameras is a hard problem showing up at DE Shaw, Goldman Sachs, eBay, Visa, and other tier-one shops. The setup sounds simple: place cameras on a binary tree so every node is either covered by a camera or adjacent to one, minimizing the number of cameras used. But the greedy instinct fails immediately. You need tree DP with three states per node, and if you haven't drilled the state transitions, you'll hit a wall in the live OA. StealthCoder solves this in seconds if you blank on the trick, invisible to the proctor.
Companies that ask "Binary Tree Cameras"
Binary Tree Cameras 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 trap is trying to greedily place cameras bottom-up or top-down. Instead, you track three states for each node: (1) uncovered and needs a camera from its parent, (2) covered by a camera at this node, (3) covered by a child's camera. DFS recurses left and right, then combines states. The recurrence is non-obvious: you can't just pick the minimum of each state independently. You have to enforce that if a node is uncovered, its parent must place a camera, and if a child places a camera, the parent is automatically covered. Mistakes pile up fast on the state logic. Tree DP problems appear across these companies frequently, and this one's accepted at under 47% because most candidates never finish the state transitions correctly. StealthCoder's value here is in the boilerplate: once you see the three-state DFS skeleton working, the pattern locks in.
Pattern tags
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Make sure you actually pass it.
Binary Tree Cameras recycles across companies for a reason. It's hard-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.
Binary Tree Cameras interview FAQ
Why doesn't a greedy bottom-up approach work?+
Greedy fails because placing a camera at one node affects coverage constraints at siblings and the parent. A local choice at node X might force expensive decisions higher up. Tree DP forces you to consider all feasible combinations, not just the locally obvious one. Binary tree problems at this level almost always need DP or memoization.
What are the three states I need to track?+
State 0: node is uncovered, needs parent's camera. State 1: node has a camera. State 2: node is covered by a child's camera. Each child contributes independently; you combine results, then enforce that if any child is uncovered, this node must have a camera.
Is this still asked at top companies?+
Yes. Goldman Sachs, Visa, eBay, and DE Shaw all report it. It's a hard problem with a 47% acceptance rate, so it filters well and shows up in final-round screens. If you see it live, prep time is tight.
How does this relate to tree DP more broadly?+
This is the canonical hard tree DP problem. It teaches state-design thinking: how to decompose a global optimization into node-local decisions. Once you nail the three-state pattern here, other tree DP problems feel more approachable. Most fall into similar decomposition patterns.
What's the biggest mistake candidates make?+
Confusing state transitions. Candidates often forget that if a child is uncovered (state 0), the parent can't also be uncovered. Also, not handling the final answer correctly: the root's valid states are only 1 or 2, never 0. Off-by-one logic in state combination kills submissions.
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