refactor: NextSmallerElement (#5412)

* refactor: NextSmallerElement

* checkstyle: fix formatting

* checkstyle: fix formatting

* checkstyle: remove redundant new line

---------

Co-authored-by: alxkm <alx@alx.com>

src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java

@@ -0,0 +1,29 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.Arguments;
+import org.junit.jupiter.params.provider.MethodSource;
+
+class NextSmallerElementTest {
+
+    @ParameterizedTest
+    @MethodSource("provideTestCases")
+    void testFindNextSmallerElements(int[] input, int[] expected) {
+        assertArrayEquals(expected, NextSmallerElement.findNextSmallerElements(input));
+    }
+
+    static Stream<Arguments> provideTestCases() {
+        return Stream.of(Arguments.of(new int[] {2, 7, 3, 5, 4, 6, 8}, new int[] {-1, 2, 2, 3, 3, 4, 6}), Arguments.of(new int[] {5}, new int[] {-1}), Arguments.of(new int[] {1, 2, 3, 4, 5}, new int[] {-1, 1, 2, 3, 4}), Arguments.of(new int[] {5, 4, 3, 2, 1}, new int[] {-1, -1, -1, -1, -1}),
+            Arguments.of(new int[] {4, 5, 2, 25}, new int[] {-1, 4, -1, 2}), Arguments.of(new int[] {}, new int[] {}));
+    }
+
+    @Test
+    void testFindNextSmallerElementsExceptions() {
+        assertThrows(IllegalArgumentException.class, () -> NextSmallerElement.findNextSmallerElements(null));
+    }
+}