Question

I’ve looked at some code for BSTs and I can see that each node is a struct. Is this necessary?

Answer 1

int flat_tree[ 1000 ][ 3 ];
    // for each tree node, value is stored in element [id][0]
                        // id of left_child stored in element [id][1]
                        // id of right_child stored in element [id][2]

…

I’m not gonna go any further with this.

Generally speaking, structs/classes are used for any kind of linked data structure. Also generally, any feature of the type system may be defeated or ignored and you can do everything (heap allocation, etc) in one array of ints in very painful fashion.