栈

public class Stack {private static class Node {private int val;private Node next;}private int N;private Node first;public Stack() {first = null;N = 0;}public void push(int value) {final Node oldFirst = first;first = new Node();first.val = value;first.next = oldFirst;N++;}public int pop() {if (isEmtpy()) {System.err.println("the stack is empty.");return -1;}final int value = first.val;first = first.next;N--;return value;}public boolean isEmtpy() {return N == 0;}@Overridepublic String toString() {final StringBuilder builder = new StringBuilder();Node node = first; while (node != null) {builder.append(node.val);builder.append(" ");node = node.next;}return builder.toString();}public static void main(String[] args) {Stack stack = new Stack();stack.push(1);stack.push(2);stack.push(3);stack.push(4);stack.push(5);stack.push(6);System.out.println(stack);stack.pop();System.out.println(stack);stack.push(7);System.out.println(stack);}}