3.2 pushとpopに加えて、最小の要素を返す関数minを持つスタックをどのようにデザインしますか？ただしpush、pop、min関数はすべてO(1)の実行時間になるようにしてください。

StackTest.java

import java.util.*;
import org.junit.*;
import static org.junit.Assert.*;

public class StackTest {
  @Test
  public void testStack(){
    Stack s = new Stack();
    s.push(1);
    s.push(2);
    s.push(3);

    assertEquals("3", String.valueOf(s.pop()));
  }

  @Test
  public void testStackReturnsMinValue(){
    Stack s = new Stack();
    s.push(3);
    assertEquals(3, s.min());
    s.push(2);
    assertEquals(2, s.min());
    s.push(1);
    assertEquals(1, s.min());
    s.pop();
    assertEquals(2, s.min());
    s.pop();
    assertEquals(3, s.min());

    s.push(1);
    assertEquals(1, s.min());
    s.push(2);
    assertEquals(1, s.min());
    s.push(3);
    assertEquals(1, s.min());
    s.pop();
    assertEquals(1, s.min());
    s.pop();
    assertEquals(1, s.min());
  }

  @Test
  public void testCounter(){
    Stack s = new Stack();
    s.push(1);

    assertEquals(1, s.count());
  }

  @Test
  public void testCounter2(){
    Stack s = new Stack();
    s.push(1);
    s.pop();

    assertEquals(0, s.count());
  }
}

class Stack {
  class Node {
    public Integer value = null;
    public Node next = null;
  }

  public Node minHead;
  public Node minTail;
  public Node head;
  public Node tail;

  public Stack(){
    this.minHead = new Node();
    this.minTail = new Node();
    this.head = new Node();
    this.tail = new Node();
    this.minHead.next = this.minTail;
    this.head.next = this.tail;
  }

  public void push(Integer val){
    Node node = new Node();
    node.value = val;
    node.next = this.head.next;
    this.head.next = node;

    if(this.minHead.next != this.minTail){
      if(this.minHead.next.value > val){
        Node min = new Node();
        min.value = val;
        min.next = this.minHead.next;
        this.minHead.next = min;
      }
    } else {
      Node min = new Node();
      min.value = val;
      this.minHead.next = min;
      min.next = this.minTail;
    }
  }

  public Integer pop(){
    if(this.head.next == this.tail){
      return null;
    } else {
      Node node = this.head.next;
      this.head.next = node.next;
      if(node.value == this.minHead.next.value){
        this.minHead.next = this.minHead.next.next;
      }
      return node.value;
    }
  }

  public int min(){
    return this.minHead.next.value;
  }

  public int count(){
    int counter = 0;
    Node node = this.head;
    while(node.next != tail){
      node = node.next;
      counter++;
    }

    return counter;
  }
}