feat: add nearest element stack algorithms with JUnit tests

Author: sgindeed

src/main/java/com/thealgorithms/stacks/NearestElement.java

@@ -0,0 +1,73 @@
+package com.thealgorithms.datastructures.stacks;
+
+import java.util.Stack;
+import java.util.Arrays;
+
+public final class NearestElement {
+
+    private NearestElement() {}
+
+    public static int[] nearestGreaterToRight(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> indexStack = new Stack<>();
+        for (int i = n - 1; i >= 0; i--) {
+            while (!indexStack.isEmpty() && arr[i] >= arr[indexStack.peek()]) {
+                indexStack.pop();
+            }
+            result[i] = indexStack.isEmpty() ? -1 : arr[indexStack.peek()];
+            indexStack.push(i);
+        }
+        return result;
+    }
+
+    public static int[] nearestGreaterToLeft(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> indexStack = new Stack<>();
+        for (int i = 0; i < n; i++) {
+            while (!indexStack.isEmpty() && arr[i] >= arr[indexStack.peek()]) {
+                indexStack.pop();
+            }
+            result[i] = indexStack.isEmpty() ? -1 : arr[indexStack.peek()];
+            indexStack.push(i);
+        }
+        return result;
+    }
+
+    public static int[] nearestSmallerToRight(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> indexStack = new Stack<>();
+        for (int i = n - 1; i >= 0; i--) {
+            while (!indexStack.isEmpty() && arr[i] <= arr[indexStack.peek()]) {
+                indexStack.pop();
+            }
+            result[i] = indexStack.isEmpty() ? -1 : arr[indexStack.peek()];
+            indexStack.push(i);
+        }
+        return result;
+    }
+
+    public static int[] nearestSmallerToLeft(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> indexStack = new Stack<>();
+        for (int i = 0; i < n; i++) {
+            while (!indexStack.isEmpty() && arr[i] <= arr[indexStack.peek()]) {
+                indexStack.pop();
+            }
+            result[i] = indexStack.isEmpty() ? -1 : arr[indexStack.peek()];
+            indexStack.push(i);
+        }
+        return result;
+    }
+
+    public static void main(String[] args) {
+        int[] sampleArray = {4, 5, 2, 10, 8};
+        System.out.println("Nearest Greater to Right: " + Arrays.toString(nearestGreaterToRight(sampleArray)));
+        System.out.println("Nearest Greater to Left: " + Arrays.toString(nearestGreaterToLeft(sampleArray)));
+        System.out.println("Nearest Smaller to Right: " + Arrays.toString(nearestSmallerToRight(sampleArray)));
+        System.out.println("Nearest Smaller to Left: " + Arrays.toString(nearestSmallerToLeft(sampleArray)));
+    }
+}

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

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+package com.thealgorithms.datastructures.stacks;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+
+public class NearestElementTest {
+
+    @Test
+    public void testNearestGreaterToRight() {
+        int[] arr = {4, 5, 2, 10, 8};
+        int[] expected = {5, 10, 10, -1, -1};
+        assertArrayEquals(expected, NearestElement.nearestGreaterToRight(arr));
+    }
+
+    @Test
+    public void testNearestGreaterToLeft() {
+        int[] arr = {4, 5, 2, 10, 8};
+        int[] expected = {-1, -1, 5, -1, 10};
+        assertArrayEquals(expected, NearestElement.nearestGreaterToLeft(arr));
+    }
+
+    @Test
+    public void testNearestSmallerToRight() {
+        int[] arr = {4, 5, 2, 10, 8};
+        int[] expected = {2, 2, -1, 8, -1};
+        assertArrayEquals(expected, NearestElement.nearestSmallerToRight(arr));
+    }
+
+    @Test
+    public void testNearestSmallerToLeft() {
+        int[] arr = {4, 5, 2, 10, 8};
+        int[] expected = {-1, 4, -1, 2, 2};
+        assertArrayEquals(expected, NearestElement.nearestSmallerToLeft(arr));
+    }
+}