test: add unit tests for StackUsingLinkedList

Author: tarandeepsingh-wd

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

@@ -0,0 +1,52 @@
+package com.thealgorithms.stacks;
+
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+public class StackUsingLinkedListTest {
+    private StackUsingLinkedList<Integer> stack;
+
+    @BeforeEach
+    public void setUp() {
+        stack = new StackUsingLinkedList<>();
+    }
+
+    @Test
+    public void testPushAndPeek() {
+        stack.push(10);
+        stack.push(20);
+
+        assertEquals(20, stack.peek());
+    }
+
+    @Test
+    public void testPop() {
+        stack.push(5);
+        stack.push(15);
+        int popped = stack.pop();
+        assertEquals(15, popped);
+        assertEquals(5, stack.peek());
+    }
+
+    @Test
+    public void testIsEmpty() {
+        assertTrue(stack.isEmpty());
+        stack.push(1);
+        assertFalse(stack.isEmpty());
+    }
+
+    @Test
+    public void testSize() {
+        assertEquals(0, stack.size());
+        stack.push(1);
+        stack.push(2);
+        assertEquals(2, stack.size());
+    }
+
+    @Test
+    public void testPopOnEmptyStack() {
+        assertThrows(RuntimeException.class, () -> stack.pop());
+    }
+}