qsort

QSort is a term used to describe sorting algorithms based on the quicksort paradigm, often implemented as an in-place, comparison-based sorter. The name appears in various programming libraries and educational materials to denote a fast general-purpose sort, frequently implemented as a function named qsort in languages such as C. While not all QSort implementations are identical, they share the core quicksort idea: select a pivot, partition the data so elements less than the pivot precede those greater than or equal to it, and then recursively sort the partitions.

In a typical QSort implementation, the array or list is reordered in place to minimize extra memory use. The partition step rearranges elements around the pivot, using swap operations. After partitioning, the algorithm recursively sorts the subarrays on either side of the pivot. This divide-and-conquer approach yields efficient average performance, but the worst case can degrade to quadratic time if pivots are poorly chosen.

Time complexity for standard quicksort-based QSort is on average O(n log n), with worst-case O(n^2). Space complexity is often O(log n) for the call stack in in-place variants. QSort is usually not stable, meaning equal elements may end up in a different relative order after sorting; some variants can be made stable with additional memory or alternate strategies.

Common optimizations include using a better pivot selection strategy (such as median-of-three), switching to insertion sort for small partitions, and employing tail recursion elimination. Some modern libraries implement hybrid or introspective versions to guarantee better worst-case behavior, while still relying on the quicksort core.