Stack

Example

Java

class Stack {
  Node top;

  Object pop() {
    if (top != null) {
      Object item = top.data;
      top = top.next;
      return item;
    }
    return null;
  }

  void push(Object item) {
    Node t = new Node(item);
    t.next = top;
    top = t;
  }

  Object peek() {
    return top.data;
  }
}