Single Pass One Line 

def hackerlandRadioTransmitters(x: Array[Int], k: Int): Int = 
    x.foldLeft((-1,-1,-1)) {
      case ((-1,-1,-1),_) => 
        x.sortInPlace
        (1,x(0),x(0)+k)
      case ((i,l,r),house) if house - k <= l => 
          (i,l,house+k)           
      case ((i,l,r),house) if house > r => 
          (i+1,house,house+k)
      case acc => acc._1
      }._1