B Tree

所有叶子节点距离根节点一定相同。

每个节点有 k 项，一定有 k+1 项叶子节点

B-Tree invariants

Because of how we add to our tree, we get two nice invariants for B-Trees:

1. All leaves must be the same distance from the source
2. A non-leaf node with k items mut has exactly k+1 children.