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Functional Style QuickSort using Java 8 and Streams
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| import java.util.stream.*; | |
| import java.util.*; | |
| /* | |
| An attempt to implement quickSort using Java 8 as close as possible to | |
| the elegant Haskell implementation: | |
| quicksort [] = [] | |
| quicksort (x:xs) = | |
| let smallerSorted = quicksort [a | a <- xs, a <= x] | |
| biggerSorted = quicksort [a | a <- xs, a > x] | |
| in smallerSorted ++ [x] ++ biggerSorted | |
| */ | |
| public class QuickSort{ | |
| public static List<Integer> quickSort(List<Integer> xs){ | |
| if(xs.size() <= 1){ | |
| return xs; | |
| } | |
| int x = xs.get(0); //choosing the first element as the pivot | |
| //smallerSorted = quicksort [a | a <- xs, a <= x] | |
| List<Integer> smallerSorted = quickSort( | |
| xs.stream() | |
| .skip(1) //removing pivot | |
| .filter(i -> i <= x) //filter the elements <= x | |
| .collect(Collectors.toList())); //convert the stream back to List | |
| //biggerSorted = quicksort [a | a <- xs, a > x] | |
| List<Integer> biggerSorted = quickSort( | |
| xs.stream() | |
| .skip(1) //removing pivot | |
| .filter(i -> i > x) //filter the elements > x | |
| .collect(Collectors.toList())); //convert the stream back to List | |
| //smallerSorted ++ [x] ++ biggerSorted | |
| return Stream.concat(Stream.concat(smallerSorted.stream(),Stream.of(x)),biggerSorted.stream()) | |
| .collect(Collectors.toList()); | |
| } | |
| public static void main(String[] args) { | |
| System.out.println(QuickSort.quickSort(new ArrayList<Integer>(Arrays.asList(3,1,2,4,5,3,4,6,7,4,6)))); //[1, 2, 3, 3, 4, 4, 4, 5, 6, 6, 7] | |
| } | |
| } |
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