Procedural approach

MissingPositive.scala

object MissingPositive extends App {

  def missingPositive(nums: Array[Int]): Int = {


    //val m1: Map[Int, Int] = (0 until nums.length).foldLeft(Map[Int, Int]())((m, i) => if (nums(i) > 0) m + (i -> 1) else m)

    val found = scala.collection.mutable.Map[Int, Int]()
    var min = Integer.MAX_VALUE

    for (n <- nums) {
      if (n >= 0) {
        found += (n -> 1)
        if (n < min) min = n
      }
    }

    if (min >= 0) {

      var count = 1
      while (found.contains(count)) {
        count += 1
      }

      count
    } else {
      min - 1
    }

  }

  println(missingPositive(Array(1, 2, 0)))
  println(missingPositive(Array(3, 4, -1, 1)))
  println(missingPositive(Array(7, 8, 9, 11, 12)))
}