leetcode202. Happy Number

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本文介绍了一种用于判断快乐数字的算法。快乐数字是指通过将数字替换为其各位数字平方之和并重复此过程直到该数变为1的过程中的数字。文章提供了一个具体的示例，即数字19如何被证实为快乐数字。

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

/**
 * @Author taotao
 * @Date 3/6/17
 * @Time O(nk) @Space O(n)
 * n为循环的数的数量，k为n的数的平均长度
 */
public class Solution {
    public boolean isHappy(int n) {
        if(n <= 0) {
            return false;
        }

        HashSet<Integer> set = new HashSet<>();
        while (set.add(n)) {
            int sum = 0;
            while (n > 0) {
                sum += (n % 10) * (n % 10);
                n = n / 10;
            }
            if (sum == 1) {
                return true;
            }
            n = sum;
        }

        return false;
    }
}