https://leetcode.com/problems/happy-number/

The key point for this question is to use a Set to keep recording
the number generated with each iteration. If we can’t add the new number
to the Set, the endless loop begins: return false. Another trick is to
get the last digit of a number, the

i = n % 10; // get the last digit
n = n / 10; // push one position right

would do that.

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public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<>();
        while (set.add(n)) {
            int total = 0;
            while (n != 0) {
                int i = n % 10;
                n /= 10;
                total += i * i;
            }
            n = total;
            if (total == 1) {
                return true;
            }
        }
        return false;
    }
}