testing: added unit tests for the MinStackUsingTwoStacks (#6387)

alxkm · DenizAltunkapan · web-flow · commit 054002adb20c · 2025-07-17T15:37:46.000Z
* testing: added unit tests for the MinStackUsingTwoStacks

* checkstyle: fix import order

---------

Co-authored-by: Deniz Altunkapan &lt;93663085+DenizAltunkapan@users.noreply.github.com&gt;
diff --git a/src/test/java/com/thealgorithms/stacks/MinStackUsingTwoStacksTest.java b/src/test/java/com/thealgorithms/stacks/MinStackUsingTwoStacksTest.java
@@ -1,38 +1,112 @@
 package com.thealgorithms.stacks;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 
+import java.util.EmptyStackException;
 import org.junit.jupiter.api.Test;
 
 public class MinStackUsingTwoStacksTest {
 
     @Test
-    public void testMinStackOperations() {
+    public void testBasicOperations() {
         MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
         minStack.push(3);
         minStack.push(5);
-        assertEquals(3, minStack.getMin());
+        assertEquals(3, minStack.getMin(), "Min should be 3");
 
         minStack.push(2);
         minStack.push(1);
-        assertEquals(1, minStack.getMin());
+        assertEquals(1, minStack.getMin(), "Min should be 1");
 
         minStack.pop();
-        assertEquals(2, minStack.getMin());
+        assertEquals(2, minStack.getMin(), "Min should be 2 after popping 1");
+
+        assertEquals(2, minStack.top(), "Top should be 2");
+    }
+
+    @Test
+    public void testPushDuplicateMins() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(2);
+        minStack.push(2);
+        minStack.push(1);
+        minStack.push(1);
+        assertEquals(1, minStack.getMin(), "Min should be 1");
+
+        minStack.pop();
+        assertEquals(1, minStack.getMin(), "Min should still be 1 after popping one 1");
+
+        minStack.pop();
+        assertEquals(2, minStack.getMin(), "Min should be 2 after popping both 1s");
+
+        minStack.pop();
+        assertEquals(2, minStack.getMin(), "Min should still be 2 after popping one 2");
+
+        minStack.pop();
+        // Now stack is empty, expect exception on getMin
+        assertThrows(EmptyStackException.class, minStack::getMin);
     }
 
     @Test
-    public void testMinStackOperations2() {
+    public void testPopOnEmptyStack() {
         MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        assertThrows(EmptyStackException.class, minStack::pop);
+    }
+
+    @Test
+    public void testTopOnEmptyStack() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        assertThrows(EmptyStackException.class, minStack::top);
+    }
+
+    @Test
+    public void testGetMinOnEmptyStack() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        assertThrows(EmptyStackException.class, minStack::getMin);
+    }
+
+    @Test
+    public void testSingleElementStack() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(10);
+        assertEquals(10, minStack.getMin());
+        assertEquals(10, minStack.top());
+
+        minStack.pop();
+        assertThrows(EmptyStackException.class, minStack::getMin);
+    }
+
+    @Test
+    public void testIncreasingSequence() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(1);
+        minStack.push(2);
         minStack.push(3);
-        minStack.push(5);
-        assertEquals(3, minStack.getMin());
+        minStack.push(4);
+
+        assertEquals(1, minStack.getMin());
+        assertEquals(4, minStack.top());
+
+        minStack.pop();
+        minStack.pop();
+        assertEquals(1, minStack.getMin());
+        assertEquals(2, minStack.top());
+    }
 
+    @Test
+    public void testDecreasingSequence() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(4);
+        minStack.push(3);
         minStack.push(2);
         minStack.push(1);
+
         assertEquals(1, minStack.getMin());
+        assertEquals(1, minStack.top());
 
         minStack.pop();
         assertEquals(2, minStack.getMin());
+        assertEquals(2, minStack.top());
     }
 }
