LeetCode刷题EASY篇Happy Number

题目

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

十分钟尝试

我的尝试思路，以前处理过int类型数据如何求出每一位，对，就是求mod运算，然后平方相加就可以。代码如下：

class Solution {
    public boolean isHappy(int n) {
        
         int res=0;
        while(true){
            while(n>0){
                int mod=n%10;
                res+=Math.pow(mod,2);
                n=n/10;  
            }
            if(res==1){
                return true;
            }
            n=res; 
        }
        return false;
    }
}

问题是无限循环，leetcode测试不通过。题目也说的是loop endlessly。不知道这个地方如何处理。看了他人对答案，用set.存储当前的和，如果能add，继续。另外，上面的代码有个问题，res应该在第一个while循环里面，修改如下，完美通过；

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