LeetCode in Kotlin

202. Happy Number

Easy

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.

Return true if n is a happy number, and false if not.

Example 1:

Input: n = 19

Output: true

Explanation: 1² + 9² = 82 8² + 2² = 68 6² + 8² = 100 1² + 0² + 0² = 1

Example 2:

Input: n = 2

Output: false

Constraints:

• 1 <= n <= 231 - 1

Solution

class Solution {
    private val set = mutableSetOf<Int>()

    tailrec fun isHappy(n: Int): Boolean {
        var num = n
        var squareSum = 0
        while (num > 0) {
            val digit = num % 10
            squareSum += digit * digit
            num /= 10
        }
        if (squareSum == 1) {
            return true
        }
        if (set.contains(squareSum)) {
            return false
        }
        set.add(squareSum)
        return isHappy(squareSum)
    }
}

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