《算法》1.3.12

1.3.12

编写一个可迭代的Stack用例，它含有一个静态的copy()方法，接受一个字符串的栈作为参数并返回该栈的一个副本。注意：这种能力是迭代器价值的一个重要体现，因为有了它我们无需改变基本API就能够实现这种功能。

强势分析一波，我们可以在Stack的API里实现一个copy方法，只需要两个栈，第一次遍历将给我们的字符串入栈，然后将得到的新的栈再次入栈，就实现了copy，返回结果即可。上代码：

/**
 * 用链表实现下压栈
 *
 * @author wangxinfu
 */
public class Stack<Item> implements Iterable<Item> {

    private int size;
    private Node first;

    private class Node {
        Item item;
        Node next;
    }

    private class StackIterator implements Iterator<Item> {
        //当前结点
        Node current = first;

        @Override
        public boolean hasNext() {
            return current != null;
        }

        @Override
        public Item next() {
            if (!hasNext()) {
                throw new NoSuchElementException("Stack is Empty!");
            }
            Item item = current.item;
            current = current.next;
            return item;
        }
    }

    /**
     * 压栈 此处无需判断isEmpty()
     * @param item
     * @return
     */
    public boolean push(Item item) {
        //在头结点添加
        Node oldFirst = first;
        first = new Node();
        first.item = item;
        first.next = oldFirst;
        size++;
        return true;
    }

    public Item pop() {
        if (isEmpty()) {
            throw new NoSuchElementException("Stack is Empty");
        }
        //暂存item
        Item item = first.item;
        Node next = first.next;
        first.item = null;
        first.next = null;
        first = next;
        size--;
        return item;
    }

    public boolean destroy(){
        while (first !=null){
            pop();
        }
        return true;
    }


    @Override
    public Iterator<Item> iterator() {
        return new StackIterator();
    }

    public int size() {
        return size;
    }

    public boolean isEmpty() {
        return first == null;
    }
    public Item top(){
        if (isEmpty()){throw new NoSuchElementException("Stack underflow");}
        return first.item;
    }

    
/**
 * 返回一个副本 
 * @param stackString
 * @return
 */
public static <T> Stack<T> copy(Stack<T> stackString){
    Stack<T>stack = new Stack<>();
    Stack<T> resStack = new Stack<>();
    for (T str:stackString){
        stack.push(str);
    }
    while (stack.iterator().hasNext()){
        resStack.push(stack.pop());
    }
        //help gc
        stack.destroy();
        return resStack;
    }
    
    
   
public static Stack<String> copyString(Stack<String>stringStack){
    return Stack.copy(stringStack);
}
 //   test
public static void main(String[]args){
    Stack<String>stack = new Stack<>();
    for (int i = 0;i<100;i++){
        stack.push(""+i);
    }
    Stack<String> copyString = copyString(stack);
    for (String str:copyString){
        System.out.println(str);
    }
    copyString.destroy();
    System.out.println(copyString.size());
}


    
}