Challenge 2: A Singleton with object — Solution

object Counter {
    var count: Int = 0

    fun increment() {
        count += 1
    }
}

fun main() {
    Counter.increment()
    Counter.increment()
    Counter.increment()
    println(Counter.count)
}

/*
Output:
3

Notes:
- Counter.increment() is called three times, each mutating the SAME
  shared count property, because "object Counter" guarantees there is
  only ever one instance of Counter in the whole program.
- With a regular class, you'd need to either create exactly one instance
  yourself and pass that same reference everywhere it's needed (easy to
  get wrong), or hand-write a singleton pattern (private constructor +
  a static-style instance holder + manual thread-safety) — object gives
  you that guarantee for free, with no boilerplate.
*/