import scala.collection.mutable
// A happy number is a number that eventually reaches 1
// when repeatedly replaced by the sum of the squares of its digits.
//
// 19 = 1^2 + 9^2 = 82
// 19 -> 82 -> 68 ->100 -> 1 eventually reaches 1
// 19 = happy numbe
object HappyNumbers {
// Compute the sum of squares of digits of n
def sumOfDigitSquares(n: Int): Int = {
var num = n
var sum = 0
while (num > 0) {
val d = num % 10
sum += d * d
num /= 10
}
sum
}
// Determine whether n is a happy number
def isHappy(n: Int): Boolean = {
val seen = mutable.Set[Int]()
var num = n
while (num != 1 && !seen.contains(num)) {
seen += num
num = sumOfDigitSquares(num)
}
num == 1
}
def main(args: Array[String]): Unit = {
var a = 1
var b = 100
if (a > b) {
val temp = a
a = b
b = temp
}
val happyNumbers = (a to b).filter(isHappy)
println(s"Happy numbers in range [$a, $b]:")
println(happyNumbers.mkString(" "))
}
}
/*
run:
Happy numbers in range [1, 100]:
1 7 10 13 19 23 28 31 32 44 49 68 70 79 82 86 91 94 97 100
*/
