using System;
using System.Collections.Generic;
// 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
class HappyNumbers
{
// Compute the sum of squares of digits of n
static int SumOfDigitSquares(int n)
{
int sum = 0;
while (n > 0) {
int d = n % 10;
sum += d * d;
n /= 10;
}
return sum;
}
// Determine whether n is a happy number
static bool IsHappy(int n)
{
HashSet<int> seen = new HashSet<int>();
while (n != 1 && !seen.Contains(n)) {
seen.Add(n);
n = SumOfDigitSquares(n);
}
return n == 1;
}
static void Main() {
int a = 1, b = 100;
if (a > b) {
int temp = a;
a = b;
b = temp;
}
List<int> happyNumbers = new List<int>();
for (int i = a; i <= b; i++) {
if (IsHappy(i)) {
happyNumbers.Add(i);
}
}
Console.WriteLine($"Happy numbers in range [{a}, {b}]:");
foreach (int n in happyNumbers) {
Console.Write(n + " ");
}
}
}
/*
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
*/
