package VL13.Aufgabe01;

/**
 * A generic stack.
 */
public class GenericStack<T> {
  StackItem<T> start;

  /**
   * A stack item to store a single item on the stack.
   */
  private class StackItem<E> {
    E value;
    StackItem<E> next;
    StackItem<E> prev;

    StackItem(E value) {
      this.value = value;
    }
  }

  /**
   * Method to push a value to the stack.
   *
   * @param value The value to store on the stack.
   */
  public void push(T value) {
    if (start == null) {
      start = new StackItem<T>(value);
    } else {
      StackItem<T> item = start;

      // get the last item of the stack.
      while (item.next != null) {
        item = item.next;
      }

      // create a new stack item to store the value.
      item.next = new StackItem<T>(value);
      item.next.prev = item;
    }
  }

  /**
   * Method to pop a value from the stack.
   *
   * @return The value from stack.
   */
  public T pop() {
    StackItem<T> item = start;

    // get the last item of the stack.
    while (item.next != null) {
      item = item.next;
    }

    // remove last item from stack.
    if (item.prev == null) {
      start = null;
    } else {
      item.prev.next = null;
    }

    // return the value of the last item of the stack.
    return item.value;
  }
}