Read a desk hierarchy level by level

Question

Given a binary tree (say, a reporting hierarchy), output its node values grouped by level: all depth-0 nodes, then depth-1, and so on, left to right within each level.

        1
       / \
      2   3
     / \
    4   5
-> [[1], [2, 3], [4, 5]]

Return a list of levels, each a left-to-right list of values.

Accepted Answer

Breadth-first search with a queue naturally visits nodes level by level. The key is to process the queue one full level at a time: record how many nodes are currently queued (that is exactly the current level's width), pop precisely that many, collect their values, and enqueue their children for the next round. Tracing the example: the queue processes [1], then [2,3], then [4,5], producing the three grouped levels. Time O(n), space O(n) for the queue (up to the widest level). Edge cases: an empty tree returns []; a single node returns [[val]]; snapshotting len(q) before the inner loop is what…

Read a desk hierarchy level by level

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