skippable iterators (jump to first value ≥ x)
* Rank: how many elements of the set are smaller than k? (counting the number of ones up to a given position)
* Select: find the kth smallest value(finding the position of the k-th bit set)
* Min/max: find the maximal and minimal value
# Navarro, Gonzalo, and Eliana Providel. "Fast, small, simple rank/select on bitmaps." In International Symposium on Experimental Algorithms, pp. 295-306. Springer Berlin Heidelberg, 2012.
# Vigna, Sebastiano. "Broadword implementation of rank/select queries." In International Workshop on Experimental and Efficient Algorithms, pp. 154-168. Springer Berlin Heidelberg, 2008.
= 基数估计 =