main.go

package main

import "fmt"

// Write an algorithm to determine if a number 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,
// and 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 numbers.
//
// Example: 19 is a happy number
//
// ^ means square
// 1^2 + 9^2 = 82
// 8^2 + 2^2 = 68
// 6^2 + 8^2 = 100
// 1^2 + 0^2 + 0^2 = 1

// brute force method
func isHappy(n int) bool {
	var history = make(map[int]struct{})
	for {
		var sum int
		for n > 0 {
			last := n % 10
			n /= 10
			sum += last * last
		}
		if sum == 1 {
			return true
		}
		if _, ok := history[sum]; ok {
			return false
		} else {
			history[sum] = struct{}{}
		}
		n = sum
	}
	return false
}

func main() {
	fmt.Println(isHappy(19))
	fmt.Println(isHappy(2))
	fmt.Println(isHappy(21))
	fmt.Println(isHappy(42))
	fmt.Println(isHappy(82))
}