That looks fine. So long as the tables are small enough to avoid memory pressure, that is probably as efficient as you need it to be.

The construction is done without waiting for I/O, all in memory. The memory used to collect the string for printing is immediately released.

What sort of efficiency do you need? If you're not sure, try profiling and benchmarking to see what is costing you. Don't optimize ahead of the facts.

After Compline,
Zaxo