java实现栈和队列

 class Node {
     int val;
     Node next;

     Node(int x) {
         val = x;
         next = null;
     }
 }

 class Stack {
     Node top;

     public Node peek() {
         if (top != null) {
             return top;
         }
         return null;
     }

     public Node pop() {
         if (top == null) {
             return null;
         } else {
             Node temp = new Node(top.val);
             top = top.next;
             return temp;
         }
     }
     public void push(Node n) {
         if (n != null) {
             n.next = top;
             top = n;
         }
     }
 }

 class Queue {
     Node first, last;

     public void enqueue(Node n) {
         if (first == null) {
             first = n;
             last = first;
         } else {
             last.next = n;
             last = n;
         }
     }

     public Node dequeue() {
         if (first == null) {
             return null;
         } else {
             Node temp = new Node(first.val);
             first = first.next;
             return temp;
         }
     }
 }