TheAlgorithms · Andruid929 · Apr 14, 2026 · Apr 14, 2026 · Apr 14, 2026 · Apr 14, 2026
@@ -1,4 +1,4 @@
-/**
+/*
  * [Brief description of what the algorithm does]
  * <p>
  * Time Complexity: O(n) [or appropriate complexity]

@@ -1,4 +1,4 @@
-/**
+/*
  * Interpolation Search estimates the position of the target value
  * based on the distribution of values.
  *

@@ -1,4 +1,4 @@
-/**
+/*
  * Performs Linear Search on an array.
  *
  * Linear search checks each element one by one until the target is found

@@ -0,0 +1,146 @@
+package com.thealgorithms.stacks;
+
+import java.util.NoSuchElementException;
+
+/**
+ * Implementation of a {@link java.util.Stack Stack} that uses a "singly" linked list
+ * to mimic the Last In First Out (LIFO) behaviour of the Stack.
+ * <p>
+ * Singly meaning the implementation only tracks the elements behind it and not both
+ * ways as the {@link java.util.LinkedList LinkedList} does.
+ * <p>
+ * <p>
+ * Supports {@link #peek()}, {@link #pop()}, {@link #push(Object) push()},
+ * checking if the structure is {@link #isEmpty()} and getting the {@link #size()}.
+ *
+ * @author Andruid929 (Andrew Jones)
+ */
+public class StackUsingLinkedList<T> {
+
+    /**
+     * The node at the top of the Stack (last inserted). If not null, contains the last inserted element
+     *
+     */
+    private Node top;
+
+    /**
+     * The size of the LinkedStack
+     *
+     */
+    private int size;
+
+    /**
+     * Creates a new instance of the Stack.
+     *
+     */
+
+    public StackUsingLinkedList() {
+    }
+
+    /**
+     * Check the element last inserted into the stack.
+     *
+     * @return the element last inserted into the Stack or {@code null} if the Stack is empty
+     *
+     */
+    public T peek() {
+        if (isEmpty()) {
+            return null;
+        }
+
+        return top.element;
+    }
+
+    /**
+     * Removes an element from the Stack and then returns the removed element.
+     *
+     * @return the element last inserted into the Stack.
+     * @throws NoSuchElementException if the Stack is empty.
+     *
+     */
+    public T pop() throws NoSuchElementException {
+        if (isEmpty()) {
+            throw new NoSuchElementException();
+        }
+
+        T removed = top.element; // Get the last inserted element before removing it
+
+        top = top.next; // Set the second to last element at the top
+
+        size--;
+
+        return removed;
+    }
+
+    /**
+     * Add a new element to the Stack.
+     *
+     * @param element the element to be added.
+     *
+     */
+    public void push(T element) {
+        Node node = new Node(element);
+
+        Node next;
+
+        if (top != null) {
+            next = top; // Save the current top node
+
+            top = node; // Update the newest node
+
+            top.next = next; // Make the new node point to the old node
+        } else {
+
+            top = node;
+        }
+
+        size++;
+    }
+
+    /**
+     * Check if the stack is empty.
+     *
+     * @return true if the Stack is empty; otherwise false.
+     *
+     */
+    public boolean isEmpty() {
+        return top == null;
+    }
+
+    /**
+     * Get the size stored within this Stack
+     *
+     * @return number of elements held in this Stack
+     *
+     */
+    public int size() {
+        return size;
+    }
+
+    /**
+     * A simple wrapper holding an element and the element that is next to it.
+     *
+     */
+    private class Node {
+
+        /**
+         * The element at this node.
+         * */
+        T element;
+
+        /**
+         * The node next to this one
+         * */
+        Node next;
+
+        /**
+         * Constructs a new node storing the given element and a null neighbour.
+         *
+         * @param element the element to be in this node.
+         * */
+        Node(T element) {
+            this.element = element;
+            this.next = null;
+        }
+    }
+}
@@ -0,0 +1,76 @@
+package com.thealgorithms.stacks;
+
+import java.util.NoSuchElementException;
+import org.junit.jupiter.api.Assertions;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class StackUsingLinkedListTest {
+
+    private StackUsingLinkedList<Integer> linkedStack;
+
+    @BeforeEach
+    public void instantiateLinkedStack() {
+        linkedStack = new StackUsingLinkedList<>();
+    }
+
+    @Test
+    void peek() {
+        Assertions.assertNull(linkedStack.peek());
+
+        linkedStack.push(10);
+        linkedStack.push(20);
+        linkedStack.push(30);
+
+        Assertions.assertEquals(30, linkedStack.peek());
+    }
+
+    @Test
+    void pop() {
+        linkedStack.push(3);
+        linkedStack.push(6);
+
+        Assertions.assertEquals(6, linkedStack.pop());
+        Assertions.assertEquals(3, linkedStack.peek());
+
+        linkedStack.pop();
+
+        Assertions.assertThrows(NoSuchElementException.class, () -> linkedStack.pop()); // Cannot pop from an empty stack
+    }
+
+    @Test
+    void push() {
+        linkedStack.push(12);
+
+        Assertions.assertEquals(12, linkedStack.peek());
+
+        linkedStack.push(15);
+        linkedStack.push(17);
+
+        Assertions.assertEquals(17, linkedStack.peek());
+    }
+
+    @Test
+    void isEmpty() {
+        Assertions.assertTrue(linkedStack.isEmpty());
+
+        linkedStack.push(1);
+
+        Assertions.assertFalse(linkedStack.isEmpty());
+    }
+
+    @Test
+    void size() {
+        linkedStack.push(1);
+        linkedStack.push(2);
+        linkedStack.push(3);
+        linkedStack.push(4);
+
+        Assertions.assertEquals(4, linkedStack.size());
+
+        linkedStack.pop();
+        linkedStack.pop();
+
+        Assertions.assertEquals(2, linkedStack.size());
+    }
+}