import java.util.HashSet;
import java.util.ArrayList;
// 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 number
public 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 boolean isHappy(int n) {
HashSet<Integer> seen = new HashSet<>();
while (n != 1 && !seen.contains(n)) {
seen.add(n);
n = sumOfDigitSquares(n);
}
return n == 1;
}
public static void main(String[] args) {
int a = 1, b = 100;
if (a > b) {
int temp = a;
a = b;
b = temp;
}
ArrayList<Integer> happyNumbers = new ArrayList<>();
for (int i = a; i <= b; i++) {
if (isHappy(i)) {
happyNumbers.add(i);
}
}
System.out.println("Happy numbers in range [" + a + ", " + b + "]:");
for (int n : happyNumbers) {
System.out.print(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
*/
