Added Next Greater Element problem in Java under stack

Java/stack/NextGreaterElement.java

@@ -0,0 +1,54 @@
+// Problem: Next Greater Element
+// Language: Java
+// Description: For each element in an array, find the next greater element to its right.
+// Uses stack to solve efficiently in O(n) time.
+
+import java.util.*;
+
+public class NextGreaterElement {
+
+    // Method to find next greater elements
+    public static int[] nextGreaterElement(int[] arr) {
+        int n = arr.length;
+        int[] result = new int[n];          // Array to store next greater elements
+        Stack<Integer> stack = new Stack<>(); // Stack to keep track of elements
+
+        for (int i = n - 1; i >= 0; i--) {
+            // Pop elements smaller than current element
+            while (!stack.isEmpty() && stack.peek() <= arr[i]) {
+                stack.pop();
+            }
+
+            // If stack is empty, no greater element to the right
+            result[i] = stack.isEmpty() ? -1 : stack.peek();
+
+            // Push current element to stack
+            stack.push(arr[i]);
+        }
+
+        return result;
+    }
+
+    // Main method to take input and print output
+    public static void main(String[] args) {
+        Scanner sc = new Scanner(System.in);
+
+        System.out.print("Enter size of array: ");
+        int n = sc.nextInt();
+
+        int[] arr = new int[n];
+        System.out.println("Enter " + n + " elements:");
+        for (int i = 0; i < n; i++) {
+            arr[i] = sc.nextInt();
+        }
+
+        int[] nge = nextGreaterElement(arr);
+
+        System.out.println("\nNext Greater Elements:");
+        for (int i = 0; i < n; i++) {
+            System.out.println(arr[i] + " -> " + nge[i]);
+        }
+
+        sc.close();
+    }
+}