Решение

class Solution {
    private val sq = intArrayOf(0,1,4,9,16,25,36,49,64,81)
    private fun next(n: Int): Int {
        var x = n
        var s = 0
        while (x != 0) { s += sq[x % 10]; x /= 10 }
        return s
    }
    fun isHappy(n: Int): Boolean {
        var slow = n
        var fast = next(n)
        while (fast != 1 && slow != fast) {
            slow = next(slow)
            fast = next(next(fast))
        }
        return fast == 1
    }
}