feat: add StackUsingLinkedList implementation with test cases

Author: prashantpiyush1111

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

@@ -0,0 +1,70 @@
+package com.thealgorithms.stacks;
+
+/**
+ * A class that implements a Stack using a singly linked list.
+ * Supports basic operations like push, pop, peek, and isEmpty.
+ *
+ * Reference: https://www.geeksforgeeks.org/stack-using-linked-list/
+ */
+public class StackUsingLinkedList {
+
+    /**
+     * Node class representing each element in the stack
+     */
+    private static class Node {
+        int data;
+        Node next;
+
+        Node(int data) {
+            this.data = data;
+        }
+    }
+
+    private Node top;
+
+    /**
+     * Push an element onto the stack
+     *
+     * @param value the value to push
+     */
+    public void push(int value) {
+        Node newNode = new Node(value);
+        newNode.next = top;
+        top = newNode;
+    }
+
+    /**
+     * Remove and return the top element of the stack
+     *
+     * @return top element
+     */
+    public int pop() {
+        if (top == null) {
+            throw new RuntimeException("Stack is empty");
+        }
+        int value = top.data;
+        top = top.next;
+        return value;
+    }
+
+    /**
+     * Return the top element without removing it
+     *
+     * @return top element
+     */
+    public int peek() {
+        if (top == null) {
+            throw new RuntimeException("Stack is empty");
+        }
+        return top.data;
+    }
+
+    /**
+     * Check if the stack is empty
+     *
+     * @return true if empty, false otherwise
+     */
+    public boolean isEmpty() {
+        return top == null;
+    }
+}

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

@@ -0,0 +1,71 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.*;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Test class for StackUsingLinkedList.
+ *
+ * This class contains unit tests to verify the correctness
+ * of stack operations such as push, pop, peek, and isEmpty.
+ *
+ * Reference: https://www.geeksforgeeks.org/stack-using-linked-list/
+ */
+class StackUsingLinkedListTest {
+
+    /**
+     * Test push and pop operations
+     */
+    @Test
+    void testPushAndPop() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        stack.push(10);
+        stack.push(20);
+
+        assertEquals(20, stack.pop());
+        assertEquals(10, stack.pop());
+    }
+
+    /**
+     * Test peek operation
+     */
+    @Test
+    void testPeek() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        stack.push(5);
+
+        assertEquals(5, stack.peek());
+    }
+
+    /**
+     * Test isEmpty method
+     */
+    @Test
+    void testIsEmpty() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+
+        assertTrue(stack.isEmpty());
+        stack.push(1);
+        assertFalse(stack.isEmpty());
+    }
+
+    /**
+     * Test pop on empty stack (edge case)
+     */
+    @Test
+    void testPopOnEmptyStack() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+
+        assertThrows(RuntimeException.class, stack::pop);
+    }
+
+    /**
+     * Test peek on empty stack (edge case)
+     */
+    @Test
+    void testPeekOnEmptyStack() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+
+        assertThrows(RuntimeException.class, stack::peek);
+    }
+}