quicksort

问题 When does the quicksort algorithm take O(n^2) time? 回答1: Quicksort works by taking a pivot, then putting all the elements lower than that pivot on one side and all the higher elements on the other; it then recursively sorts the two sub groups in the same way (all the way down until everything is sorted.) Now if you pick the worst pivot each time (the highest or lowest element in the list) you'll only have one group to sort, with everything in that group other than the original pivot that you

问题 When does the quicksort algorithm take O(n^2) time? 回答1: Quicksort works by taking a pivot, then putting all the elements lower than that pivot on one side and all the higher elements on the other; it then recursively sorts the two sub groups in the same way (all the way down until everything is sorted.) Now if you pick the worst pivot each time (the highest or lowest element in the list) you'll only have one group to sort, with everything in that group other than the original pivot that you

问题 I found this sample code online, which explains how the qsort function works. I could not understand what the compare function returns. #include "stdlib.h" int values[] = { 88, 56, 100, 2, 25 }; int cmpfunc (const void * a, const void * b) //what is it returning? { return ( *(int*)a - *(int*)b ); //What is a and b? } int main(int argc, _TCHAR* argv[]) { int n; printf("Before sorting the list is: \n"); for( n = 0 ; n < 5; n++ ) { printf("%d ", values[n]); } qsort(values, 5, sizeof(int),

问题 I am trying to work out an efficient quicksort algo. It works okay, but takes long time to run when the number of elements are huge, and certain sections of the array are pre-sorted. I was looking up the Wikipedia article on quicksort , and there I found this written: To make sure at most O(log N) space is used, recurse first into the smaller half of the array, and use a tail call to recurse into the other. Use insertion sort, which has a smaller constant factor and is thus faster on small

问题 How can I implement a concurrent quicksort or mergesort algorithm for Java? We've had issues on a 16-(virtual)-cores Mac where only one core (!) was working using the default Java sorting algo and it was, well, not good to see that very fine machine be completely underused. So we wrote our own (I wrote it) and we did indeed gain good speedups (I wrote a multithreaded quicksort and due to its partitioning nature it parallelize very well but I could have written a mergesort too)... But my

问题 I am trying to implement an QuickSort algorithm on an ArrayList. However, I am getting a Exception in thread "main" java.lang.StackOverflowError at sorting.QuickSort.quickSort(QuickSort.java:25) at sorting.QuickSort.quickSort(QuickSort.java:36) at sorting.QuickSort.quickSort(QuickSort.java:36) at sorting.QuickSort.quickSort(QuickSort.java:36) at sorting.QuickSort.quickSort(QuickSort.java:36) ... I am not sure as to why is there an overflow. Below is my implementation: public static void

问题 Im trying to create a quick sort algorithm using pointers and am having a but of trouble The line: int* pivot = partition(start, stop); is causing the error "No matching function for call to partition". There may be other issues with the code, but this is the one not allowing me to run. Any help would be greatly appreciated. void quickSort(int* start, int* stop) { if (stop - start <= 1) return; int* pivot = partition(start, stop); quickSort(start, pivot); quickSort(pivot + 1, stop); } int

问题 Currently, I have a barebones implementation of the quicksort algorithm to sort some randomly generated numbers. The sort is efficient, more so than merge sort. However, for specific number sets (e.g. reversed numbers that need to be sorted the other way around), I need to optimized the pivot. int* partition (int* first, int* last); void quickSort(int* first, int* last) { if (last - first <= 1) return; int* pivot = partition(first, last); quickSort(first, pivot); quickSort(pivot + 1, last); }

问题 I've seen some code in Google implementing Quicksort algorithm like this. static public void QuickSort_Recursive(int [] arr, int left, int right) { // For Recusrion if(left < right) { int pivot = Partition(arr, left, right); if(pivot > 1) QuickSort_Recursive(arr, left, pivot - 1); if(pivot + 1 < right) QuickSort_Recursive(arr, pivot + 1, right); } } I tried to work out with this, I've already understood how the code itself works but one thing I got confused. How the recursion (the code above)

问题 Why we say the bestcase for quicksort is that "each time we perform a partition we divide the list into two nearly equal pieces"? And how to prove this is exactly the so called "best case"? 回答1: Take array of length 2^N (for simplicity). Compare number of operations for the case of perfect partitions at every stage ( N into N/2+N/2 ) and for the case of division of segment length N into 1 and N-1 回答2: I created a program rather than trying to do an analysis. I compared the case of 1/2, 1/2