Nodes hold 1, 2, or 3 keys (2-, 3-, or 4-node). Insertion uses top-down preemptive splitting: any 4-node encountered on the way down is split before we step into it, guaranteeing the parent always has room. Directly isomorphic to a red-black tree.
Insert
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Keys
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Depth
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4-Nodes
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3-Nodes
Try it: Load "Pre-Split Demo" to see when 4-nodes form and when the next insert pre-splits them on the way down. Insert is always a single downward pass, splits never propagate back up.