Add MonotonicStack class with element retrieval methods

Author: NayanSaraff

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

@@ -0,0 +1,55 @@
+import java.util.*;
+
+public class MonotonicStack {
+
+    // Returns Next Greater Element for each element in the array
+    public static int[] nextGreaterElement(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> stack = new Stack<>(); // stores indices
+
+        for (int i = n - 1; i >= 0; i--) {
+            // Pop elements smaller or equal to arr[i]
+            while (!stack.isEmpty() && arr[i] >= arr[stack.peek()]) {
+                stack.pop();
+            }
+
+            // If stack is empty, no greater element to the right
+            result[i] = stack.isEmpty() ? -1 : arr[stack.peek()];
+
+            // Push current index onto stack
+            stack.push(i);
+        }
+
+        return result;
+    }
+
+    // Returns Next Smaller Element for each element in the array
+    public static int[] nextSmallerElement(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];
+        Stack<Integer> stack = new Stack<>(); // stores indices
+
+        for (int i = n - 1; i >= 0; i--) {
+            // Pop elements greater or equal to arr[i]
+            while (!stack.isEmpty() && arr[i] <= arr[stack.peek()]) {
+                stack.pop();
+            }
+
+            result[i] = stack.isEmpty() ? -1 : arr[stack.peek()];
+            stack.push(i);
+        }
+
+        return result;
+    }
+
+    public static void main(String[] args) {
+        int[] arr = {4, 5, 2, 10, 8};
+
+        int[] nextGreater = nextGreaterElement(arr);
+        int[] nextSmaller = nextSmallerElement(arr);
+
+        System.out.println("Next Greater Element: " + Arrays.toString(nextGreater));
+        System.out.println("Next Smaller Element: " + Arrays.toString(nextSmaller));
+    }
+}