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Group a tree by vertical columns

MetaSoftware Engineer · Technical Screen
MediumSoftware EngineerTechnical ScreenCoding & Algorithms

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This interview question evaluates algorithm design, data structures, correctness, complexity, edge cases, and implementation details in a realistic interview setting. A strong answer for Group a tree by vertical columns states assumptions, handles edge cases, explains trade-offs, and shows how to validate the result clearly.

Given the root of a binary tree, produce a vertical listing of nodes: assign each node an x-coordinate (root at x=0; left child x−1; right child x+ 1). For each x from smallest to largest, list node values from top to bottom; when multiple nodes share the same (x, depth), order them by their left-to-right appearance in level order. Return a list of columns (each a list of values). Describe your traversal strategy, tie-breaking, and data structures. Analyze time and space complexity.