202. 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

这道题的注意点是如何判断无限循环。这里采用hash set，add()方法返回一个boolean值，若所加的值已存在，则出现了循环。

public class Solution {
    public boolean isHappy(int n) {
        Set<Integer> set = new HashSet<Integer>();
        while(set.add(n)) {
            int squareDigits = 0;
            while(n > 0) {
                squareDigits += Math.pow(n % 10, 2);
                n /= 10;
            }
            if(squareDigits == 1) return true;
            n = squareDigits;
        }
        return false;
    }
}