Describe the bug
This is not a sparse merkle tree.

Sparse merkle trees definitionally have 2**n nodes at each level of the tree (where n is the 0-indexed depth). If you intend to build a sparse merkle, you ought not pad.

If you intend to build a regular merkle that allows incremental storage of nodes, you ought not use the default hash mechanism (bytes32(height) would be sufficient padding).

Also, please consider tagging your interior nodes

Expected behavior
Name should reflect common definition of sparse merkles