use node index access instead of pointers

[lucabart97]

I see that different tree search libraries uses in the struct Node not the child pointers but the Node index in a vector, to improve performance and be cache friendly.
Do you think that this can be possible? What do you expect about the performance result?

[jlblancoc]

I can't recall it right now, but I think it was benchmarked to be faster as is. However, your idea is worth investigating...