Biased 2-3 trees

We describe a new data structure for maintaining collections of weighted items. The access time for an item of weight w in a collection of total weight W is proportional to log(W/w) in the worst case (which is optimal in a certain sense), and several other useful operations can be made to work just, as fast. The data structure is simpler than previous proposals, but the running time must be amortized over a sequence of operations to achieve the time bounds.