Added the retrieval of minimum and maximum element from stack at O(1)

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

@@ -0,0 +1,74 @@
+package com.thealgorithms.stacks;
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+/**
+ * A class that implements a stack that gives the maximum element in O(1) time.
+ * The mainStack is used to store the all the elements of the stack
+ * While the maxStack stores the maximum elements
+ * When we want to get a maximum element, we call the top of the maximum stack
+ *
+ * Problem: https://leetcode.com/problems/min-stack/
+ */
+public class GreatestElementConstantTime {
+    private Stack<Integer> mainStack;
+    private Stack<Integer> maxStack;
+
+    /**
+     * Constructs two empty stacks
+     */
+    public GreatestElementConstantTime() {
+        mainStack = new Stack<>();
+        maxStack = new Stack<>();
+    }
+
+    /**
+     * Pushes an element onto the top of the stack.
+     * Checks if the element is the maximum or not
+     * If so, then pushes to the maximum stack
+     * @param data The element to be pushed onto the stack.
+     */
+    public void push(int data) {
+        if (mainStack.isEmpty()) {
+            mainStack.push(data);
+            maxStack.push(data);
+            return;
+        }
+
+        mainStack.push(data);
+        if (data > maxStack.peek()) {
+            maxStack.push(data);
+        }
+    }
+
+    /**
+     * Pops an element from the stack.
+     * Checks if the element to be popped is the maximum or not
+     * If so, then pop from the minStack
+     *
+     * @throws NoSuchElementException if the stack is empty.
+     */
+    public void pop() {
+        if (mainStack.isEmpty()) {
+            throw new NoSuchElementException("Stack is empty");
+        }
+
+        int ele = mainStack.pop();
+        if (ele == maxStack.peek()) {
+            maxStack.pop();
+        }
+    }
+
+    /**
+     * Returns the maximum element present in the stack
+     *
+     * @return The element at the top of the maxStack, or null if the stack is empty.
+     */
+    public Integer getMaximumElement() {
+        if (maxStack.isEmpty()) {
+            return null;
+        }
+        return maxStack.peek();
+    }
+}

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

@@ -0,0 +1,72 @@
+package com.thealgorithms.stacks;
+
+import java.util.NoSuchElementException;
+import java.util.Stack;
+
+/**
+ * A class that implements a stack that gives the minimum element in O(1) time.
+ * The mainStack is used to store the all the elements of the stack
+ * While the minStack stores the minimum elements
+ * When we want to get a minimum element, we call the top of the minimum stack
+ */
+public class SmallestElementConstantTime {
+    private Stack<Integer> mainStack;
+    private Stack<Integer> minStack;
+
+    /**
+     * Constructs two empty stacks
+     */
+    public SmallestElementConstantTime() {
+        mainStack = new Stack<>();
+        minStack = new Stack<>();
+    }
+
+    /**
+     * Pushes an element onto the top of the stack.
+     * Checks if the element is the minimum or not
+     * If so, then pushes to the minimum stack
+     * @param data The element to be pushed onto the stack.
+     */
+    public void push(int data) {
+        if (mainStack.isEmpty()) {
+            mainStack.push(data);
+            minStack.push(data);
+            return;
+        }
+
+        mainStack.push(data);
+        if (data < minStack.peek()) {
+            minStack.push(data);
+        }
+    }
+
+    /**
+     * Pops an element from the stack.
+     * Checks if the element to be popped is the minimum or not
+     * If so, then pop from the minStack
+     *
+     * @throws NoSuchElementException if the stack is empty.
+     */
+    public void pop() {
+        if (mainStack.isEmpty()) {
+            throw new NoSuchElementException("Stack is empty");
+        }
+
+        int ele = mainStack.pop();
+        if (ele == minStack.peek()) {
+            minStack.pop();
+        }
+    }
+
+    /**
+     * Returns the minimum element present in the stack
+     *
+     * @return The element at the top of the minStack, or null if the stack is empty.
+     */
+    public Integer getMinimumElement() {
+        if (minStack.isEmpty()) {
+            return null;
+        }
+        return minStack.peek();
+    }
+}

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

@@ -0,0 +1,70 @@
+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class GreatestElementConstantTimeTest {
+
+    private GreatestElementConstantTime constantTime;
+
+    @BeforeEach
+    public void setConstantTime() {
+        constantTime = new GreatestElementConstantTime();
+    }
+
+    @Test
+    public void testMaxAtFirst() {
+        constantTime.push(1);
+        constantTime.push(10);
+        constantTime.push(20);
+        constantTime.push(5);
+        assertEquals(20, constantTime.getMaximumElement());
+    }
+
+    @Test
+    public void testMinTwo() {
+        constantTime.push(5);
+        constantTime.push(10);
+        constantTime.push(20);
+        constantTime.push(1);
+        assertEquals(20, constantTime.getMaximumElement());
+        constantTime.pop();
+        constantTime.pop();
+        assertEquals(10, constantTime.getMaximumElement());
+    }
+
+    @Test
+    public void testNullMax() {
+        constantTime.push(10);
+        constantTime.push(20);
+        constantTime.pop();
+        constantTime.pop();
+        assertNull(constantTime.getMaximumElement());
+    }
+
+    @Test
+    public void testBlankHandle() {
+        constantTime.push(10);
+        constantTime.push(1);
+        constantTime.pop();
+        constantTime.pop();
+        assertThrows(NoSuchElementException.class, () -> constantTime.pop());
+    }
+
+    @Test
+    public void testPushPopAfterEmpty() {
+        constantTime.push(10);
+        constantTime.push(1);
+        constantTime.pop();
+        constantTime.pop();
+        constantTime.push(5);
+        assertEquals(5, constantTime.getMaximumElement());
+        constantTime.push(1);
+        assertEquals(5, constantTime.getMaximumElement());
+    }
+}

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

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+package com.thealgorithms.stacks;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+public class SmallestElementConstantTimeTest {
+
+    private SmallestElementConstantTime sect;
+
+    @BeforeEach
+    public void setSect() {
+        sect = new SmallestElementConstantTime();
+    }
+
+    @Test
+    public void testMinAtFirst() {
+        sect.push(1);
+        sect.push(10);
+        sect.push(20);
+        sect.push(5);
+        assertEquals(1, sect.getMinimumElement());
+    }
+
+    @Test
+    public void testMinTwo() {
+        sect.push(5);
+        sect.push(10);
+        sect.push(20);
+        sect.push(1);
+        assertEquals(1, sect.getMinimumElement());
+        sect.pop();
+        assertEquals(5, sect.getMinimumElement());
+    }
+
+    @Test
+    public void testNullMin() {
+        sect.push(10);
+        sect.push(20);
+        sect.pop();
+        sect.pop();
+        assertNull(sect.getMinimumElement());
+    }
+
+    @Test
+    public void testBlankHandle() {
+        sect.push(10);
+        sect.push(1);
+        sect.pop();
+        sect.pop();
+        assertThrows(NoSuchElementException.class, () -> sect.pop());
+    }
+
+    @Test
+    public void testPushPopAfterEmpty() {
+        sect.push(10);
+        sect.push(1);
+        sect.pop();
+        sect.pop();
+        sect.push(5);
+        assertEquals(5, sect.getMinimumElement());
+        sect.push(1);
+        assertEquals(1, sect.getMinimumElement());
+    }
+}