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Adds quick select and quick top algos

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This commit is contained in:
Abhinav Sarkar 2019-07-08 20:55:20 +05:30
parent 66973cf46d
commit 70e2911852
2 changed files with 101 additions and 15 deletions
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24
src/main/java/net/abhinavsarkar/algorist/Sorter.java
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@ -4,6 +4,30 @@ public interface Sorter<T>
{
T[] sort(T[] input, Comparator<T> comparator);
default T select(T[] input, int rank, Comparator<T> comparator) {
if (rank < 1) {
throw new IllegalArgumentException("rank is less than one");
}
if (rank > input.length) {
throw new IllegalArgumentException("rank is larger than input length");
}
T[] sorted = sort(input, comparator);
return sorted[rank - 1];
}
default T[] top(T[] input, int rank, Comparator<T> comparator) {
if (rank < 1) {
throw new IllegalArgumentException("rank is less than one");
}
if (rank > input.length) {
throw new IllegalArgumentException("rank is larger than input length");
}
return sort(input, comparator);
}
@FunctionalInterface
interface Comparator<T> {
int compare(T o1, T o2);

92
src/main/java/net/abhinavsarkar/algorist/sort/QuickSorter.java
View File

@ -6,7 +6,6 @@ import net.abhinavsarkar.algorist.Sorter;
public class QuickSorter<T> implements Sorter<T>
{
private static final ThreadLocalRandom RANDOM = ThreadLocalRandom.current();
@Override
public T[] sort(T[] input, Comparator<T> comparator)
@ -19,14 +18,6 @@ public class QuickSorter<T> implements Sorter<T>
return sort(input, 0, input.length, comparator);
}
private void shuffle(T[] input)
{
for (int i = input.length - 1; i > 0; i--) {
int j = RANDOM.nextInt(i + 1);
swap(input, i, j);
}
}
private T[] sort(T[] input, int start, int end, Comparator<T> comparator)
{
if (end - start <= 1) {
@ -39,6 +30,76 @@ public class QuickSorter<T> implements Sorter<T>
return input;
}
@Override
public T[] top(T[] input, int rank, Comparator<T> comparator)
{
checkRank(input, rank);
shuffle(input);
return top(input, rank - 1, 0, input.length, comparator);
}
private T[] top(T[] input, int rank, int start, int end, Comparator<T> comparator)
{
if (end - start <= 1) {
return input;
}
int pivot = partition(input, start, end, comparator);
sort(input, start, pivot, comparator);
if (pivot <= rank)
{
top(input, rank, pivot, end, comparator);
}
return input;
}
@Override
public T select(T[] input, int rank, Comparator<T> comparator)
{
checkRank(input, rank);
shuffle(input);
return select(input, rank - 1, 0, input.length, comparator);
}
public void checkRank(T[] input, int rank)
{
if (rank < 1) {
throw new IllegalArgumentException("rank is less than one");
}
if (rank > input.length) {
throw new IllegalArgumentException("rank is larger than input length");
}
}
private T select(T[] input, int rank, int start, int end, Comparator<T> comparator)
{
if (end - start == 1) {
if (rank == start) {
return input[start];
}
throw new IllegalStateException("Impossible");
}
int pivot = partition(input, start, end, comparator);
if (pivot == rank) {
return input[pivot];
} else if (pivot > rank) {
return select(input, rank, start, pivot, comparator);
} else {
return select(input, rank, pivot, end, comparator);
}
}
private void shuffle(T[] input)
{
for (int i = input.length - 1; i > 0; i--) {
int j = ThreadLocalRandom.current().nextInt(i + 1);
swap(input, i, j);
}
}
private int partition(T[] input, int start, int end, Comparator<T> comparator)
{
int pivot = end - 1;
@ -68,12 +129,13 @@ public class QuickSorter<T> implements Sorter<T>
public static void main(String[] args)
{
Sorter<String> sorter = new QuickSorter<>();
String[] sorted =
sorter.sort(new String[]{
"abhinav", "sarkar", "barista", "jordan", "data",
"cata", "meta", "dota", "best", "recursion"},
(s, anotherString) -> anotherString.compareTo(s));
System.out.println(Arrays.toString(sorted));
String[] input = {"abhinav", "sarkar", "barista", "jordan", "data", "cata",
"meta", "dota", "best", "recursion"};
Comparator<String> comparator = (s1, s2) -> s2.compareTo(s1);
System.out.println(Arrays.toString(sorter.sort(input, comparator)));
System.out.println(sorter.select(input, 1, comparator));
System.out.println(Arrays.toString(sorter.top(input, 3, comparator)));
}
}