# Happy Number

Write an algorithm to determine if a number n is happy. A happy number is a number defined by the following process:

• Starting with any positive integer, replace the number by the sum of the squares of its digits. Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy.

Source

package questions

import _utils.UseCommentAsDocumentation
import kotlin.test.assertFalse
import kotlin.test.assertTrue

/**
* Write an algorithm to determine if a number n is happy.
* A happy number is a number defined by the following process:
* * Starting with any positive integer, replace the number by the sum of the squares of its digits.
* Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1.
* Those numbers for which this process ends in 1 are happy.
*
* [Source](https://leetcode.com/problems/happy-number)
*/
@UseCommentAsDocumentation
private fun isHappy(n: Int): Boolean {
if (n == 1) return true
var digit = n
val seen = mutableSetOf<Int>()
while (digit != 1) {
digit = sumOfSquareDigits(digit.toString())
if (digit == 1) {
return true
} else if (seen.contains(digit)) {
return false
}
}
return false
}

private fun sumOfSquareDigits(n: String): Int {
var result = 0
for (i in n) {
val numericValue = Character.getNumericValue(i)
result += numericValue * numericValue
}
return result
}

fun main() {
assertTrue(isHappy(1))

// For 19:
// 12 + 92 = 82
//82 + 22 = 68
//62 + 82 = 100
//12 + 02 + 02 = 1
assertTrue(isHappy(19))

assertFalse(isHappy(2))
}

Updated on 2021-10-13