LeetCode-Happy Number

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Description：
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:

Input: 19
Output: true

Explanation:
12 + 92 = 82
82 + 22 = 68
62 + 82 = 100
12 + 02 + 02 = 1

题意：判断一个正数是否是个“Happy Number”。定义是将这个数的每一位上的数平方求和重新赋给这个数，直到这个数的值为1或者产生一个循环停止；

解法：解决这道题目的关键是如何判断终止条件，有两种情况我们会退出循环；第一种就是判断平方后的值为1即可；第二种我们需要判断是否产生了一个循环，可以将每次求得的平方和存储在哈希表中，这样，只要我们检测到哈希表中已经存在这个数了，那么说明已经产生了循环，就可以退出循环了；

Java

class Solution {
    public boolean isHappy(int n) {
        Set<Integer> table = new HashSet<>();
        while (!(n == 1 || table.contains(n))) {
            int key = n;
            n = getNextNum(n);
            table.add(key);
        }
        return n == 1 ? true : false;
    }

    private int getNextNum(int n) {
        int sum = 0;
        while (n > 0) {
            sum += Math.pow(n % 10, 2);
            n /= 10;
        }
        return sum;
    }
}