```java package ds.impl; import ds.util.Node; import ds.Stack; import java.util.Iterator; import ds.util.ListIterator; public class LinkedStack<T> implements Iterable<T>,Stack<T> { private Node<T> first; private int size; public LinkedStack() { first = null; size = 0; } @Override public int size() { return size; } /** * 向棧中推入一個新元素 * * @param data 新元素 */ @Override public void push(T data) { Node<T> oldFirst = first; first = new Node<>(); first.data = data; first.next = oldFirst; size++; } /** * 從棧中彈出棧首元素 * * @return 棧首元素 */ @Override public T pop() { notEmptyCheck(); T result = first.data; first = first.next; size--; return result; } /** * 查看棧首元素 * @return 棧首元素 */ @Override public T peek() { notEmptyCheck(); return first.data; } @Override public String toString() { StringBuilder s = new StringBuilder(); for (T data : this) { s.append(data).append(' '); } return s.toString(); } @Override public Iterator<T> iterator() { return new ListIterator(first); } } ```