Add the retrieval of minimum and maximum element from stack at O(1) (#5714)

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://www.baeldung.com/cs/stack-constant-time
+ */
+public class GreatestElementConstantTime {
+    private Stack<Integer> mainStack; // initialize a mainStack
+    private Stack<Integer> maxStack; // initialize a 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 +1,74 @@
+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
+ *
+ * Problem: https://www.baeldung.com/cs/stack-constant-time
+ */
+public class SmallestElementConstantTime {
+    private Stack<Integer> mainStack; // initialize a mainStack
+    private Stack<Integer> minStack; // initialize a 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();
+    }
+}