Add MinStackUsingSingleStack algorithm (#5759)

DIRECTORY.md

@@ -431,6 +431,7 @@
             * [StrobogrammaticNumber](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/StrobogrammaticNumber.java)
             * [SumOfArithmeticSeries](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/SumOfArithmeticSeries.java)
             * [SumOfDigits](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/SumOfDigits.java)
+            * [SumOfOddNumbers](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/SumOfOddNumbers.java)
             * [SumWithoutArithmeticOperators](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/SumWithoutArithmeticOperators.java)
             * [TrinomialTriangle](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/TrinomialTriangle.java)
             * [TwinPrime](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/maths/TwinPrime.java)
@@ -612,6 +613,7 @@
             * [InfixToPrefix](https://github.com/TheAlgorithms/Java/blob/master/src/main/java/com/thealgorithms/stacks/InfixToPrefix.java)
             * [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)
             * [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)
@@ -1009,6 +1011,7 @@
             * [StrobogrammaticNumberTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/StrobogrammaticNumberTest.java)
             * [SumOfArithmeticSeriesTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/SumOfArithmeticSeriesTest.java)
             * [SumOfDigitsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/SumOfDigitsTest.java)
+            * [SumOfOddNumbersTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/SumOfOddNumbersTest.java)
             * [SumWithoutArithmeticOperatorsTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/SumWithoutArithmeticOperatorsTest.java)
             * [TestArmstrong](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/TestArmstrong.java)
             * [TwinPrimeTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/maths/TwinPrimeTest.java)
@@ -1164,6 +1167,7 @@
             * [InfixToPostfixTest](https://github.com/TheAlgorithms/Java/blob/master/src/test/java/com/thealgorithms/stacks/InfixToPostfixTest.java)
             * [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)
             * [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/MinStackUsingSingleStack.java

@@ -0,0 +1,65 @@
+package com.thealgorithms.stacks;
+
+import java.util.EmptyStackException;
+import java.util.Stack;
+
+/**
+ * Min-Stack implementation using a single stack.
+ *
+ * This stack supports push, pop, and retrieving the minimum element
+ * in constant time (O(1)) using a modified approach where the stack
+ * stores both the element and the minimum value so far.
+ *
+ * @author Hardvan
+ */
+public class MinStackUsingSingleStack {
+    private final Stack<long[]> stack = new Stack<>();
+
+    /**
+     * Pushes a new value onto the stack.
+     * Each entry stores both the value and the minimum value so far.
+     *
+     * @param value The value to be pushed onto the stack.
+     */
+    public void push(int value) {
+        if (stack.isEmpty()) {
+            stack.push(new long[] {value, value});
+        } else {
+            long minSoFar = Math.min(value, stack.peek()[1]);
+            stack.push(new long[] {value, minSoFar});
+        }
+    }
+
+    /**
+     * Removes the top element from the stack.
+     */
+    public void pop() {
+        if (!stack.isEmpty()) {
+            stack.pop();
+        }
+    }
+
+    /**
+     * Retrieves the top element from the stack.
+     *
+     * @return The top element of the stack.
+     */
+    public int top() {
+        if (!stack.isEmpty()) {
+            return (int) stack.peek()[0];
+        }
+        throw new EmptyStackException();
+    }
+
+    /**
+     * Retrieves the minimum element in the stack.
+     *
+     * @return The minimum element so far.
+     */
+    public int getMin() {
+        if (!stack.isEmpty()) {
+            return (int) stack.peek()[1];
+        }
+        throw new EmptyStackException();
+    }
+}

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

@@ -0,0 +1,66 @@
+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 MinStackUsingSingleStackTest {
+
+    @Test
+    public void testBasicOperations() {
+        MinStackUsingSingleStack minStack = new MinStackUsingSingleStack();
+
+        minStack.push(3);
+        minStack.push(5);
+        assertEquals(3, minStack.getMin(), "Minimum should be 3");
+
+        minStack.push(2);
+        minStack.push(1);
+        assertEquals(1, minStack.getMin(), "Minimum should be 1");
+
+        minStack.pop();
+        assertEquals(2, minStack.getMin(), "Minimum should be 2");
+
+        minStack.pop();
+        assertEquals(3, minStack.getMin(), "Minimum should be 3");
+    }
+
+    @Test
+    public void testTopElement() {
+        MinStackUsingSingleStack minStack = new MinStackUsingSingleStack();
+
+        minStack.push(8);
+        minStack.push(10);
+        assertEquals(10, minStack.top(), "Top element should be 10");
+
+        minStack.pop();
+        assertEquals(8, minStack.top(), "Top element should be 8");
+    }
+
+    @Test
+    public void testGetMinAfterPops() {
+        MinStackUsingSingleStack minStack = new MinStackUsingSingleStack();
+
+        minStack.push(5);
+        minStack.push(3);
+        minStack.push(7);
+
+        assertEquals(3, minStack.getMin(), "Minimum should be 3");
+
+        minStack.pop(); // Popping 7
+        assertEquals(3, minStack.getMin(), "Minimum should still be 3");
+
+        minStack.pop(); // Popping 3
+        assertEquals(5, minStack.getMin(), "Minimum should now be 5");
+    }
+
+    @Test
+    public void testEmptyStack() {
+        MinStackUsingSingleStack minStack = new MinStackUsingSingleStack();
+
+        assertThrows(EmptyStackException.class, minStack::top, "Should throw exception on top()");
+        assertThrows(EmptyStackException.class, minStack::getMin, "Should throw exception on getMin()");
+    }
+}