feat: Add MinStackUsingTwoStacks new algorithm with Junit tests (TheAlgorithms#5758)

Author: Hardvan

DIRECTORY.md

@@ -614,6 +614,7 @@
             * [LargestRectangle](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/LargestRectangle.java)
             * [MaximumMinimumWindow](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MaximumMinimumWindow.java)
             * [MinStackUsingSingleStack](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MinStackUsingSingleStack.java)
+            * [MinStackUsingTwoStacks](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/MinStackUsingTwoStacks.java)
             * [NextGreaterElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextGreaterElement.java)
             * [NextSmallerElement](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/NextSmallerElement.java)
             * [PostfixEvaluator](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/PostfixEvaluator.java)
@@ -1168,6 +1169,7 @@
             * [InfixToPrefixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPrefixTest.java)
             * [LargestRectangleTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/LargestRectangleTest.java)
             * [MinStackUsingSingleStackTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/MinStackUsingSingleStackTest.java)
+            * [MinStackUsingTwoStacksTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/MinStackUsingTwoStacksTest.java)
             * [NextGreaterElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextGreaterElementTest.java)
             * [NextSmallerElementTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/NextSmallerElementTest.java)
             * [PostfixEvaluatorTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/PostfixEvaluatorTest.java)

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

@@ -0,0 +1,57 @@
+package com.thealgorithms.stacks;
+
+import java.util.Stack;
+
+/**
+ * Min-Stack implementation that supports push, pop, and retrieving the minimum element in constant time.
+ *
+ * @author Hardvan
+ */
+public final class MinStackUsingTwoStacks {
+    MinStackUsingTwoStacks() {
+    }
+
+    private final Stack<Integer> stack = new Stack<>();
+    private final Stack<Integer> minStack = new Stack<>();
+
+    /**
+     * Pushes a new element onto the {@code stack}.
+     * If the value is less than or equal to the current minimum, it is also pushed onto the {@code minStack}.
+     *
+     * @param value The value to be pushed.
+     */
+    public void push(int value) {
+        stack.push(value);
+        if (minStack.isEmpty() || value <= minStack.peek()) {
+            minStack.push(value);
+        }
+    }
+
+    /**
+     * Removes the top element from the stack.
+     * If the element is the minimum element, it is also removed from the {@code minStack}.
+     */
+    public void pop() {
+        if (stack.pop().equals(minStack.peek())) {
+            minStack.pop();
+        }
+    }
+
+    /**
+     * Retrieves the top element of the stack.
+     *
+     * @return The top element.
+     */
+    public int top() {
+        return stack.peek();
+    }
+
+    /**
+     * Retrieves the minimum element in the stack.
+     *
+     * @return The minimum element.
+     */
+    public int getMin() {
+        return minStack.peek();
+    }
+}

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

@@ -0,0 +1,38 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+import org.junit.jupiter.api.Test;
+
+public class MinStackUsingTwoStacksTest {
+
+    @Test
+    public void testMinStackOperations() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(3);
+        minStack.push(5);
+        assertEquals(3, minStack.getMin());
+
+        minStack.push(2);
+        minStack.push(1);
+        assertEquals(1, minStack.getMin());
+
+        minStack.pop();
+        assertEquals(2, minStack.getMin());
+    }
+
+    @Test
+    public void testMinStackOperations2() {
+        MinStackUsingTwoStacks minStack = new MinStackUsingTwoStacks();
+        minStack.push(3);
+        minStack.push(5);
+        assertEquals(3, minStack.getMin());
+
+        minStack.push(2);
+        minStack.push(1);
+        assertEquals(1, minStack.getMin());
+
+        minStack.pop();
+        assertEquals(2, minStack.getMin());
+    }
+}