// 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
// Compute the sum of squares of digits of n
fun sumOfDigitSquares(n: Int): Int {
var num = n
var sum = 0
while (num > 0) {
val d = num % 10
sum += d * d
num /= 10
}
return sum
}
// Determine whether n is a happy number
fun isHappy(n: Int): Boolean {
val seen = mutableSetOf<Int>()
var num = n
while (num != 1 && num !in seen) {
seen.add(num)
num = sumOfDigitSquares(num)
}
return num == 1
}
fun main() {
var a = 1
var b = 100
if (a > b) {
val temp = a
a = b
b = temp
}
val happyNumbers = mutableListOf<Int>()
for (i in a..b) {
if (isHappy(i)) {
happyNumbers.add(i)
}
}
println("Happy numbers in range [$a, $b]:")
println(happyNumbers.joinToString(" "))
}
/*
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
*/
