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Quicksort must store a constant amount of information for each nested recursive call. Since the best case makes at most O(log n) nested recursive calls, it uses O(log n) space. However, without Sedgewick's trick to limit the recursive calls, in the worst case quicksort could make O(n) nested recursive calls and need O(n) auxiliary space.
Efficient implementations of quicksort (with in-place partitioning) are typically unstable sorts and somewhat complex but are among the fastest sorting algorithms in practice. Together with its modest O(log n) space usage, quicksort is one of the most popular sorting algorithms and is available in many standard programming libraries.
For example, the best case for a simple linear search on a list occurs when the desired element is the first element of the list. Development and choice of algorithms is rarely based on best-case performance: most academic and commercial enterprises are more interested in improving average-case complexity and worst-case performance. Algorithms ...
Performing a Fast Fourier transform; heapsort, quicksort (best and average case), or merge sort quadratic: Multiplying two n-digit numbers by a simple algorithm; bubble sort (worst case or naive implementation), Shell sort, quicksort , selection sort or insertion sort (), >
Third, average-case complexity allows discriminating the most efficient algorithm in practice among algorithms of equivalent best case complexity (for instance Quicksort). Average-case analysis requires a notion of an "average" input to an algorithm, which leads to the problem of devising a probability distribution over inputs.
In the best case, which occurs when the input is already sorted, it runs in linear time, meaning that it is an adaptive sorting algorithm. [ 3 ] It is superior to Quicksort for sorting object references or pointers because these require expensive memory indirection to access data and perform comparisons and Quicksort's cache coherence benefits ...
The best-case for the algorithm is when the numbers are already sorted, which takes () steps to perform the task. However, the input in the worst-case for the algorithm is when the numbers are reverse sorted and it takes O ( n 2 ) {\displaystyle O(n^{2})} steps to sort them; therefore the worst-case time-complexity of insertion sort is of O ( n ...
Are there quicksort implementations which push the worst case performance below O(n 2)? AxelBoldt Well, the STL uses the "introspection sort", which basically uses a median-of-three pivot, and switches to a true O(n log n) sort if the recursion depth is greater than k*log n, for some k, typically 2.