fix: changing counting sort to keep it with standard implementation

Author: Alex Klymenko

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

@@ -0,0 +1,54 @@
+package com.thealgorithms.sorts;
+
+import java.util.Arrays;
+
+/**
+ * A standard implementation of the Counting Sort algorithm for integer arrays.
+ * This implementation has a time complexity of O(n + k), where n is the number
+ * of elements in the input array and k is the range of the input.
+ * It works only with integer arrays.
+ *
+ * The space complexity is O(k), where k is the range of the input integers.
+ *
+ * Note: This implementation does not handle negative integers as it
+ * calculates the range based on the minimum and maximum values of the array.
+ *
+ */
+public final class CountingSort {
+    private CountingSort() {
+    }
+
+    /**
+     * Sorts an array of integers using the Counting Sort algorithm.
+     *
+     * @param array the array to be sorted
+     * @return the sorted array
+     */
+    public static int[] sort(int[] array) {
+        if (array.length == 0) {
+            return array;
+        }
+
+        int max = Arrays.stream(array).max().orElse(Integer.MIN_VALUE);
+        int min = Arrays.stream(array).min().orElse(Integer.MAX_VALUE);
+        int range = max - min + 1;
+
+        int[] count = new int[range];
+        int[] output = new int[array.length];
+
+        for (int value : array) {
+            count[value - min]++;
+        }
+
+        for (int i = 1; i < count.length; i++) {
+            count[i] += count[i - 1];
+        }
+
+        for (int i = array.length - 1; i >= 0; i--) {
+            output[count[array[i] - min] - 1] = array[i];
+            count[array[i] - min]--;
+        }
+
+        return output;
+    }
+}

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

@@ -0,0 +1,26 @@
+package com.thealgorithms.sorts;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
+import java.util.stream.Stream;
+
+public class CountingSortTest {
+
+    record TestCase(int[] inputArray, int[] expectedArray) {
+    }
+
+    static Stream<TestCase> provideTestCases() {
+        return Stream.of(new TestCase(new int[] {}, new int[] {}), new TestCase(new int[] {4}, new int[] {4}), new TestCase(new int[] {6, 1, 99, 27, 15, 23, 36}, new int[] {1, 6, 15, 23, 27, 36, 99}), new TestCase(new int[] {6, 1, 27, 15, 23, 27, 36, 23}, new int[] {1, 6, 15, 23, 23, 27, 27, 36}), new TestCase(new int[] {5, 5, 5, 5, 5},
+            new int[] {5, 5, 5, 5, 5}), new TestCase(new int[] {1, 2, 3, 4, 5}, new int[] {1, 2, 3, 4, 5}), new TestCase(new int[] {5, 4, 3, 2, 1}, new int[] {1, 2, 3, 4, 5}), new TestCase(new int[] {3, -1, 4, 1, 5, -9}, new int[] {-9, -1, 1, 3, 4, 5}), new TestCase(new int[] {0, 0, 0, 0}, new int[] {0, 0, 0, 0}),
+            new TestCase(new int[] {3, 3, -1, -1, 2, 2, 0, 0}, new int[] {-1, -1, 0, 0, 2, 2, 3, 3}), new TestCase(new int[] {-3, -2, -1, -5, -4}, new int[] {-5, -4, -3, -2, -1}), new TestCase(new int[] {1000, 500, 100, 50, 10, 5, 1}, new int[] {1, 5, 10, 50, 100, 500, 1000}));
+    }
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    public void testCountingSort(TestCase testCase) {
+        int[] outputArray = CountingSort.sort(testCase.inputArray);
+        assertArrayEquals(testCase.expectedArray, outputArray);
+    }
+}