"test: add comprehensive tests for StackUsingLinkedList"

Author: gowtham1412-p

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

@@ -0,0 +1,69 @@
+package com.thealgorithms.datastructures.stacks;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.*;
+
+class StackUsingLinkedListTest {
+    
+    @Test
+    void testPushAndPop() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        stack.push(10);
+        stack.push(20);
+        stack.push(30);
+        
+        assertEquals(30, stack.pop());
+        assertEquals(20, stack.pop());
+        assertEquals(10, stack.pop());
+    }
+    
+    @Test
+    void testPeek() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        stack.push(100);
+        stack.push(200);
+        
+        assertEquals(200, stack.peek());
+        assertEquals(200, stack.peek());
+    }
+    
+    @Test
+    void testIsEmpty() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        assertTrue(stack.isEmpty());
+        
+        stack.push(5);
+        assertFalse(stack.isEmpty());
+        
+        stack.pop();
+        assertTrue(stack.isEmpty());
+    }
+    
+    @Test
+    void testSize() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        assertEquals(0, stack.size());
+        
+        stack.push(1);
+        assertEquals(1, stack.size());
+        
+        stack.push(2);
+        stack.push(3);
+        assertEquals(3, stack.size());
+        
+        stack.pop();
+        assertEquals(2, stack.size());
+    }
+    
+    @Test
+    void testPopEmptyStack() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        assertThrows(RuntimeException.class, () -> stack.pop());
+    }
+    
+    @Test
+    void testPeekEmptyStack() {
+        StackUsingLinkedList stack = new StackUsingLinkedList();
+        assertThrows(RuntimeException.class, () -> stack.peek());
+    }
+}