fix(sorts): handle small arrays in FlashSort and empty arrays in DutchNationalFlagSort

src/main/java/com/thealgorithms/sorts/DutchNationalFlagSort.java

@@ -13,6 +13,9 @@ public class DutchNationalFlagSort implements SortAlgorithm {
 
     @Override
     public <T extends Comparable<T>> T[] sort(T[] array) {
+        if (array.length == 0) {
+            return array;
+        }
         return dutchNationalFlagSort(array, array[(int) Math.ceil((array.length) / 2.0) - 1]);
     }
 

src/main/java/com/thealgorithms/sorts/FlashSort.java

@@ -76,7 +76,8 @@ private <T extends Comparable<? super T>> void flashSort(T[] arr) {
             return; // All elements are the same
         }
 
-        final int m = (int) (classificationRatio * arr.length);
+        // At least one class, otherwise small arrays index into an empty classification table
+        final int m = Math.max(1, (int) (classificationRatio * arr.length));
 
         final int[] classificationArray = new int[m];
 

src/test/java/com/thealgorithms/sorts/DutchNationalFlagSortTest.java

@@ -109,4 +109,10 @@ void testOddStringsMidGivenDnfs() {
         dutchNationalFlagSort.sort(strings, "e");
         assertArrayEquals(strings, stringsResult);
     }
+
+    @Test
+    void testEmptyArray() {
+        Integer[] integers = {};
+        assertArrayEquals(new Integer[] {}, new DutchNationalFlagSort().sort(integers));
+    }
 }

src/test/java/com/thealgorithms/sorts/FlashSortTest.java

@@ -1,5 +1,6 @@
 package com.thealgorithms.sorts;
 
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
@@ -59,6 +60,12 @@ public List<DynamicTest> dynamicTestsForSorting() {
         return dynamicTests;
     }
 
+    @Test
+    public void testArraysTooSmallForMoreThanOneClass() {
+        assertArrayEquals(new Integer[] {1, 2}, new FlashSort().sort(new Integer[] {2, 1}));
+        assertArrayEquals(new Integer[] {1, 2, 3, 4, 5}, new FlashSort(0.1).sort(new Integer[] {5, 3, 1, 4, 2}));
+    }
+
     private List<DynamicTest> createDynamicTestsForRatio(double ratio) {
         List<DynamicTest> dynamicTests = new ArrayList<>();
         for (TestMethod testMethod : getTestMethodsFromSuperClass()) {