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;
        }
    } 
}