Happy Number

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

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

解题思路：

这题看似很复杂，其实不太难。唯一的难点，题目已经给出了，就是如果结局不是1的话，必然是一个loop。那么我们只要用一个set记录所有计算过的结果就行了。如果set重复了，并且没有1，就返回false。

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<Integer>();
        int result = squaresDigits(n);
        while(result != 1 && !set.contains(result)) {
            set.add(result);
            result = squaresDigits(result);
        }
        if(result == 1) {
            return true;
        }
        return false;
    }

    public int squaresDigits(int n) {
        int result = 0;
        while(n > 0) {
            int lastDigit = n % 10;
            result = result + lastDigit * lastDigit;
            n = n / 10;
        }
        return result;
    }
}