1 class Node { 2     int val; 3     Node next; 4      5     Node(int x) { 6         val = x; 7         next = null; 8     } 9 }10 11 class Stack {12     Node top;13     14     public Node peek() {15         if (top != null) {16             return top;17         }18         return null;19     }20     21     public Node pop() {22         if (top == null) {23             return null;24         } else {25             Node temp = new Node(top.val);26             top = top.next;27             return temp;28         }29     }30     public void push(Node n) {31         if (n != null) {32             n.next = top;33             top = n;34         }35     }36 }37 38 class Queue {39     Node first, last;40     41     public void enqueue(Node n) {42         if (first == null) {43             first = n;44             last = first;45         } else {46             last.next = n;47             last = n;48         }49     }50     51     public Node dequeue() {52         if (first == null) {53             return null;54         } else {55             Node temp = new Node(first.val);56             first = first.next;57             return temp;58         }59     } 60 }