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The point of example is to show that graphs (for Airbnb case - trees) are a good fit. Modern sql database should be enough, but just because it has strong indexing, which is mostly implemented using B-trees, which are again graphs. So Airbnb example takes the reader to balanced binary trees (though it seems long reading).

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I thought it was odd to treat trees as graphs. They are, but they're different enough that you don't use the same tools on them.

Hmm? Trees are just acyclic and connected graphs in graph theory. Why wouldn't you use the same tools that apply to graphs in general?

Acyclicity usually makes it possible to come up with a specialized algorithm that works better on trees than an algorithm that needs to handle all possible graphs. For example, many NP-complete problems become fixed-parameter tractable if you fix the tree-width of the graph (essentially, how many nodes you have to bag together until the graph looks like a tree).

Because of this, algorithms on trees are often very different from general graph algorithms in practice. You could use the same tools, but it would be unnecessarily slow and/or complex.

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