> For the complete documentation index, see [llms.txt](https://blog.bernardw.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://blog.bernardw.com/problems/o-1-stack.md).

# O(1) Stack

How would you design a stack which, in addition, to push and pop, also has a function min which returns the minimum element?

Push, pop and min should all operate in O(1) time.

{% tabs %}
{% tab title="Scala" %}

```scala
/**
  * 3.2
  * @param value
  * @param min
  */
case class Frame(value: Int, min: Int)

class Stack {
  var elems:List[Frame] = Nil
  def push(elem: Int): Unit = this.elems = this.elems match {
    case Nil => Frame(elem, elem) :: this.elems
    case _ => Frame(elem, elems.head.min min elem) :: this.elems
  }
  def pop: Int = this.elems match {
    case Nil => -1
    case x :: xs => {
      this.elems = xs
      x.value
    }
  }
  def min: Int = this.elems match {
    case Nil => -1
    case x :: xs => x.min
  }
}

val s = new Stack
s.push(5)
s.push(3)
s.push(99)
s.min
s.pop
s.pop
s.min
```

{% endtab %}

{% tab title="Second Tab" %}

{% endtab %}
{% endtabs %}


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