#include <iostream>
#include <unordered_set>
#include <vector>
// 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
// Compute the sum of squares of digits of n
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
bool isHappy(int n) {
std::unordered_set<int> seen;
while (n != 1 && !seen.count(n)) {
seen.insert(n);
n = sumOfDigitSquares(n);
}
return n == 1;
}
int main() {
int a = 1, b = 100;
if (a > b) std::swap(a, b);
std::vector<int> happyNumbers;
for (int i = a; i <= b; ++i) {
if (isHappy(i)) {
happyNumbers.push_back(i);
}
}
std::cout << "Happy numbers in range [" << a << ", " << b << "]:\n";
for (int n : happyNumbers) {
std::cout << 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
*/
