Implement a O(1) stack

Author: cszach

src/main/lists/Stack.java

@@ -0,0 +1,59 @@
+/**
+ * An implementation of a stack based on a linked list.
+ *
+ * <p>
+ * The head of the linked list is the top of the stack. All operations are O(1)
+ * in time and space complexity. The length (or size) of the stack is not stored
+ * and thus cannot be retrieved in constant time.
+ *
+ * <p>
+ * This implementation also does not contain any if statement thanks to two
+ * techniques: not storing the length (as described above) and having a tail
+ * whose <code>next</code> pointer points to itself.
+ *
+ * <p>
+ * Operations with their time complexities are:
+ *   <ul>
+ *     <li><code>push</code>: O(1)</li>
+ *     <li><code>pop</code>: O(1)</li>
+ *     <li><code>peek</code>: O(1)</li>
+ *   </ul>
+ */
+public class Stack<T> {
+  private LinkedListNode<T> head;
+
+  public Stack() {
+    LinkedListNode<T> tail = new LinkedListNode<T>(null, null);
+    tail.next = tail;
+
+    this.head = new LinkedListNode<T>(null, tail);
+  }
+
+  public Stack(T[] data) {
+    LinkedListNode<T> tail = new LinkedListNode<T>(null, null);
+    tail.next = tail;
+
+    this.head = new LinkedListNode<T>(null, new LinkedListNode<T>(null, null));
+
+    for (T d: data)
+      this.push(d);
+  }
+
+  public boolean isEmpty() {
+    return this.head.next == this.head.next.next;
+  }
+
+  public Stack<T> push(T data) {
+    this.head.next = new LinkedListNode<T>(data, this.head.next);
+    return this;
+  }
+
+  public Stack<T> pop() {
+    this.head.next = this.head.next.next;
+    return this;
+  }
+
+  public T peek() {
+    return this.head.next.data;
+  }
+}