DSW algorithm

DSW algorithm

The DSW algorithm, or in full Day/Stout/Warren algorithm, is a method for efficiently balancing binary search trees — that is, decreasing their height to O(log n) nodes, where n is the total number of nodes. Unlike a self-balancing binary search tree, it does not do this incrementally during each operation, but periodically, so that its cost can be amortized over many operations. The algorithm was designed by Quentin F. Stout and Bette Warren in their 1986 paper Tree Rebalancing in Optimal Time and Space, based on work done by Colin Day in 1976.

The algorithm requires linear (O(n)) time and is in-place. The original algorithm by Day generates as compact a tree as possible: all levels of the tree are completely full except possibly the bottom-most. The Stout/Warren modification generates a complete binary tree, namely one in which the bottom-most level is filled strictly from left to right. This is a useful transformation to perform if it is known that no more inserts will be done.

A 2002 article by Timothy J. Rolfe has recently brought attention back to the DSW algorithm after a long hiatus; the naming is from the section title "6.7.1: The DSW Algorithm" in Adam Drozdek's Data Structures and Algorithms in C++ (PWS Publishing Co., 1996) pp. 173-175. Rolfe cites two main advantages: "in circumstances in which one generates an entire binary search tree at the beginning of processing, followed by item look-up access for the rest of processing" and "pedagogically within a course on data structures where one progresses from the binary search tree into self-adjusting trees, since it gives a first exposure to doing rotations within a binary search tree."

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