Happy Number——LeetCode

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

• 1² + 9² = 82
• 8² + 2² = 68
• 6² + 8² = 100
• 1² + 0² + 0² = 1

LeetCode新题，题目大意：给定一个数，将其每一位平方和相加得到一个新的数，以此循环往复，最后得到的数如果是1，则给定的数是happy number，否则会无尽的循环。

解题思路：一开始直接模拟，循环100次还不是1直接剪掉return false，也能AC，但是毕竟可能有情况覆盖不到，改用HastSet存放中间数就行了，如果得到的新结果在中间数集合中出现了，那么一定会陷入循环并且得到的不是1。

Talk is cheap>>

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