When we descend from a node to its child, you need to map positions.
If you're tracking range at a node, and we go left (to elements with bit 0):
- New left index:
- New right index:
Going right (to elements with bit 1):
- New left index:
- New right index:
This mapping is the core of all wavelet tree queries. It tells us which positions in the child correspond to your query range.