链表生成栈和队列

结点类

class Node {
        int val;
        Node next;

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

栈

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;
            }
        }
    }

队列

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;
            }

        }
    }