Description of Quicksort
Quicksort is a divide-and-conquer sorting algorithm (see Chapter 1). It is widely regarded as the best for general use. Like insertion sort, it is a comparison sort that sorts in place, but its efficiency makes it a better choice for medium to large sets of data.
Returning to the example of sorting a pile of canceled checks by hand, we begin with an unsorted pile that we partition in two. In one pile we place all checks numbered less than or equal to what we think may be the median value, and in the other pile we place the checks greater than this. Once we have the two piles, we divide each of them in the same manner, and we repeat the process until we end up with one check in every pile. At this point, the checks are sorted.
Since quicksort is a divide-and-conquer algorithm, it is helpful to consider it more formally in terms of the three steps common to all divide-and-conquer algorithms:
Divide: partition the data into two partitions around a partition value.
Conquer: sort the two partitions by recursively applying quicksort to them.
Combine: do nothing since the partitions are sorted after the previous step.
Considering its popularity, it may be surprising that the worst case of quicksort is no better than the worst case of insertion sort. However, with a little care we can make the worst case of quicksort so unlikely that we can actually count on the algorithm performing to its average case, which is considerably better. The key to reliably achieving quicksort’s ...